kernel: ar8216: add get_features method

Modifying PHY capabilities in the probe function broke with upstream
commit 92ed2eb7f4b7 ("net: phy: probe the PHY before determining the
supported features").

AR8316 switches only support 10/100 Mbit/s link modes because of this
change.

Provide a get_features method for the PHY driver, so Gigabit link mode
will be advertised to link partners again.

Signed-off-by: David Bauer <mail@david-bauer.net>