net, phy: fix AR8031 phy_mask

AR8035 driver will be never applied because of wrong mask for
AR8031 driver. Fix this.

Signed-off-by: Heiko Schocher <hs@denx.de>
Reported-by: Pavel Nakonechny <pavel.nakonechny@skitlab.ru>
Cc: Andy Fleming <afleming@freescale.com>
Cc: Joe Hershberger <joe.hershberger@gmail.com>
Patch: 276944