i915/gvt: remove hardcoded value on crc32_start calculation

i915/gvt: remove hardcoded value on crc32_start calculation

struct gvt_firmware_header has a crc32 member in which all members that
come after the that field are used to calculate it. The previous
implementation added the value '4' (crc32's u32 size) to calculate the
crc32_start offset which came across as a bit cryptic until you take a
deeper look at the struct.

This patch changes crc32_start offset to the 'version' member which is
the first member of the struct gvt_firmware_header after crc32.

It's worth mentioning that doing a build before/after this patch results
in no binary output differences.

Signed-off-by: Paulo Miguel Almeida <paulo.miguel.almeida.rodenas@gmail.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20221030033628.GA279284@mail.google.com
Reviewed-by: Zhenyu Wang <zhenyuw@linux.intel.com>