drm/amdgpu: remove set but not used variable 'header'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/gpu/drm/amd/amdgpu/amdgpu_ucode.c: In function 'amdgpu_ucode_init_bo':
drivers/gpu/drm/amd/amdgpu/amdgpu_ucode.c:431:39: warning:
 variable 'header' set but not used [-Wunused-but-set-variable]

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>