ar8216: factor out PHY init code into a generic function

PHY init code in the switch-specific hw_init functions is mainly
identical. Factor it out into a generic ar8xxx_phy_init function.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Felix Fietkau <nbd@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@43331 3c298f89-4303-0410-b956-a3cf2f4a3e73