drm/amdgpu: fix overflowed constant warning in mmhub_set_clockgating()

To fix potential overflowed constant warning, modify the variables to u32
for getting the return value of RREG32_SOC15().

Signed-off-by: Bob Zhou <bob.zhou@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>