drm/radeon: fix endian bugs in radeon_atom_get_clock_dividers() (v3)

v2: fix copy paste typo.
v3: clarify new union member

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>