net: dsa: rtl8366rb: Support setting STP state

This adds support for setting the STP state to the RTL8366RB
DSA switch. This rids the following message from the kernel on
e.g. OpenWrt:

DSA: failed to set STP state 3 (-95)

Since the RTL8366RB has one STP state register per FID with
two bit per port in each, we simply loop over all the FIDs
and set the state on all of them.

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