realtek: pcs: reject SerDes modes unsupported by the instance

realtek: pcs: reject SerDes modes unsupported by the instance

Add a per-SerDes supported_modes bitmap, filled at probe by each
variant from the SerDes id or type, and reject unsupported modes in
determine_hw_mode() via test_bit().

This replaces the rtl838x is_hw_mode_supported() switch and adds the
same gating to rtl839x/rtl93xx, which previously relied only on the
per-variant mode-value table. Unlike that table, the bitmap is per
SerDes instance, so it also rejects modes the table can encode but that
a given SerDes cannot actually use. Rejection uses the uapi -EOPNOTSUPP
instead of the internal -ENOTSUPP.

Link: https://github.com/openwrt/openwrt/pull/23608
Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>