net/phy: Fix the phy id mask of AR8031

The both AR8031 and AR8035 belong to Atheros 803x serial PHY.
So the phy id mask of AR8031 is the same to the phy id mask
of AR8035. The right mask value is 0x4fffff.

This patch has been tested on the P1010 and P1023.

Signed-off-by: Chunhe Lan <Chunhe.Lan@freescale.com>
Cc: Joe Hershberger <joe.hershberger@gmail.com>
Patch: 287748