drm/amdgpu/soc15: fix warnings in register macro

expects argument of type ‘unsigned int’ has type ‘long int’

Fixes: 52e211c1

 ("drm/amdgpu:Add error message when register failed to reach expected value")
Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: James Zhu <James.Zhu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>