generic: ar8216: improve ar8xxx_is_possible check

The commit "generic: ar8216: add sanity check to ar8216_probe"
(774da6c7) stated that PHY IDs
should be checked at address 0-4. However, the PHY 4 was
never check by the loop. This patch extends the check to be
similar to the Atheors SDK. It tries all 4 ports and skips
unconnected PHYs if necessary. If it cannot find any familiar
PHYs, it will prevent the phy driver from initializing.

This patch is necessary for the C-60. It doesn't have a
PHY at port 3, so this caused the check in ar8xxx_is_possible
to fail. As a result, the ethernet ports on the C-60 didn't
work.

Signed-off-by: Christian Lamparter <chunkeey@gmail.com>