drm/amdgpu: remove redundant NULL check

release_firmware() checks whether firmware pointer is NULL.
Remove the redundant NULL check in psp_sw_fini().

Signed-off-by: Yushan Zhou <katrinzhou@tencent.com>
Reviewed-by: Luben Tuikov <luben.tuikov@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>