realtek: rtl93xx: Add learning and flooding enable/disable

realtek: rtl93xx: Add learning and flooding enable/disable

Both RTL930x and RTL931x were missing the code to support enabling and
disabling MAC address learning and unknown unicast flooding on a per-port
basis.

* rtl93*x_enable_learning() allows toggling of dynamic MAC learning on
  individual ports by modifying the L2 learning constraint control
  register.
* rtl93*x_enable_flood() provides the ability to control unknown unicast
  flooding behavior, disabling forwarding when set. If it is enabled, it
  will just forward it. If it is disabled, packets will simply be dropped.

Signed-off-by: Harshal Gohel <hg@simonwunderlich.de>
Signed-off-by: Sharadanand Karanjkar <sk@simonwunderlich.de>
Link: https://github.com/openwrt/openwrt/pull/19581
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>