drm/radeon: properly unregister hwmon interface (v2)

Need to properly unregister the hwmon device on driver
unload.

v2: minor clean up

bug:
https://bugzilla.kernel.org/show_bug.cgi?id=73931



Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org