net: dsa: rtl8366rb: Support disabling learning

The RTL8366RB hardware supports disabling learning per-port
so let's make use of this feature. Rename some unfortunately
named registers in the process.

Suggested-by: Vladimir Oltean <olteanv@gmail.com>
Cc: Alvin Šipraga <alsi@bang-olufsen.dk>
Cc: Mauri Sandberg <sandberg@mailfence.com>
Cc: Florian Fainelli <f.fainelli@gmail.com>
Cc: DENG Qingfang <dqfext@gmail.com>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>