ar8216: introduce global constant for number of PHYs

All supported switches have 5 PHYs. Currently partially 5 is hardcoded
and partially switch-specific constants exist.
Replace them with a global constant.

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

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@43328 3c298f89-4303-0410-b956-a3cf2f4a3e73