[AVR32] Initialize phy_mask for both macb devices

The STK1000 uses pullups on the MDIO lines to the PHY, but they are
too weak. This causes the PHY layer to detect PHYs on all possible MII
addresses. Mask out all but the correct address to prevent this from
happening.

Signed-off-by: Haavard Skinnemoen <hskinnemoen@atmel.com>