drm/radeon/dpm: cleanup a type issue with rv6xx_clocks_per_unit()

The rv6xx_clocks_per_unit() function pretends it can set flags in a u64
bitfield but really because "1" is an int it doesn't work for more than
32 bits.  The only caller truncates the high bits away anyway.  I've
just changed it to be a u32.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>