ramips: mt7620: fix external PHY autopolling

The port initialisation is based on assumption that phy address and
port number is the same. SoC allow different numbers and some board
have it.

Use phy address instead the port number to make sure that correct
addresses are polled.

In situation when only one PHY with address 0x0 is conected to
port 4, autopolling is broken.

This patch make autopolling correct when port number and phy address
are different.

Signed-off-by: Pawel Dembicki <paweldembicki@gmail.com>