generic: ar8216: add support for get_port_stats()

generic: ar8216: add support for get_port_stats()

Partially reverts commit eff3549c5883a9abc5dbff00c084cabbcfdf4437.

AR7240 and AR9341 have buggy hardware switch LED trigger. The AR7240
one doesn't blink and the blinking of port0/port5 is reversed on
AR9341 if we swap PHY0 and PHY4. (Only blinking is reversed, which
means LED for PHY0 will lit when PHY0 is link up and will blink when
PHY4 has active link and vice versa.) On these two chips a software
swconfig LED trigger is required.

This commit adds swconfig port stats back but:
 1. move checking of mib_t/rxb_id into ar8xxx_chip since we can't
    distinguish ar7240sw and ar8216 using only chip id.
 2. don't update mib counter in get_port_stat. This function is called
    every 0.01s and this capturing procedure will take up a lot of CPU.
    We already have a mib_work_func updating mib counters every 2s so
    return the saved counter instead of fetching new data. The blinking
    rate will be weird but it should solve the previously mentioned CPU
    time problem.

Signed-off-by: Chuanhong Guo <gch981213@gmail.com>