drm/amdgpu: Fix snprintf buffer size in smu_v14_0_init_microcode

drm/amdgpu: Fix snprintf buffer size in smu_v14_0_init_microcode

This commit addresses buffer overflow in the smu_v14_0_init_microcode
function. The issue was about the snprintf function writing more bytes
into the fw_name buffer than it can hold.

The line of code is:

snprintf(fw_name, sizeof(fw_name), "amdgpu/%s.bin", ucode_prefix);

Here, snprintf is used to write a formatted string into fw_name. The
format is "amdgpu/%s.bin", where %s is a placeholder for the string
ucode_prefix. The sizeof(fw_name) argument tells snprintf the maximum
number of bytes it can write into fw_name, including the
null-terminating character. In the original code, fw_name is an array of
30 characters.

The string "amdgpu/%s.bin" could be up to 41 bytes long, which exceeds
the 30 bytes allocated for fw_name. This is because %s could be replaced
by ucode_prefix, which can be up to 29 characters long. Adding the 12
characters from "amdgpu/" and ".bin", the total length could be 41
characters.

To address this, the size of ucode_prefix has been reduced to 15
characters. This ensures that the maximum length of the string written into
fw_name does not exceed its capacity.

smu_13/14 etc. don't follow legacy scheme ie., amdgpu_ucode_legacy_naming

Fixes the below with gcc W=1:
drivers/gpu/drm/amd/amdgpu/../pm/swsmu/smu14/smu_v14_0.c: In function ‘smu_v14_0_init_microcode’:
drivers/gpu/drm/amd/amdgpu/../pm/swsmu/smu14/smu_v14_0.c:80:52: warning: ‘%s’ directive output may be truncated writing up to 29 bytes into a region of size 23 [-Wformat-truncation=]
   80 |         snprintf(fw_name, sizeof(fw_name), "amdgpu/%s.bin", ucode_prefix);
      |                                                    ^~       ~~~~~~~~~~~~
drivers/gpu/drm/amd/amdgpu/../pm/swsmu/smu14/smu_v14_0.c:80:9: note: ‘snprintf’ output between 12 and 41 bytes into a destination of size 30
   80 |         snprintf(fw_name, sizeof(fw_name), "amdgpu/%s.bin", ucode_prefix);
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Fixes: fe6cd9152464 ("drm/amd/swsmu: add smu14 ip support")
Cc: Li Ma <li.ma@amd.com>
Cc: Likun Gao <Likun.Gao@amd.com>
Cc: Lijo Lazar <lijo.lazar@amd.com>
Cc: Kenneth Feng <kenneth.feng@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Christian König <christian.koenig@amd.com>
Signed-off-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>
Suggested-by: Lijo Lazar <lijo.lazar@amd.com>
Reviewed-by: Lijo Lazar <lijo.lazar@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>