net: dsa: qca8k: convert to GENMASK/FIELD_PREP/FIELD_GET

Convert and try to standardize bit fields using
GENMASK/FIELD_PREP/FIELD_GET macros. Rework some logic to support the
standard macro and tidy things up. No functional change intended.

Signed-off-by: Ansuel Smith <ansuelsmth@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>