hw/mips/malta: Fix minor dead code issue

hw/mips/malta: Fix minor dead code issue

Coverity points out (in CID 1508390) that write_bootloader has
some dead code, where we assign to 'p' and then in the following
line assign to it again. This happened as a result of the
refactoring in commit cd5066f8618b.

Fix the dead code by removing the 'void *v' variable entirely and
instead adding a cast when calling bl_setup_gt64120_jump_kernel(), as
we do at its other callsite in write_bootloader_nanomips().

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>