ar8216: display flow control info in swconfig get_link in case of autonegatiation too

The swconfig get_link attribute (at least) on AR8327/AR8337 doesn't
consider the autonegotiated flow control.
AR8327/AR8337 provide the info about autonegotiated rx/tx flow control
in bits 10 and 11 of the port status register.
Use these values to display info about autonegotiated rx/tx flow
control as part of the get_link attribute.

Successfully tested on TL-WDR4900 (AR8327 rev.4) and
TL-WDR4300 (AR8327 rev.2).

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@44023 3c298f89-4303-0410-b956-a3cf2f4a3e73