realtek: pcs: rtl931x: fix and cleanup CMU functions

realtek: pcs: rtl931x: fix and cleanup CMU functions

Fix the wrong values bit values when setting CMU band which were the
same for both 'enable == true' and 'enable == false'.

While at it, fix some coding issues in the CMU functions:
- drop confusing debug output
- use ternary value instead of if-else
- return proper error
- make variable declaration in reverse christmas tree
- drop unneeded temporary value

Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/21707
Signed-off-by: Robert Marko <robimarko@gmail.com>