phy/micrel: Change phy_id_mask for KSZ8721

There are KSZ8721 PHYs with phy_id 0x00221619. In order to detect them
as PHY_ID_KSZ8001 compatible while staying different to PHY_ID_KSZ9021
ignore the last two bits when matching PHY_ID

Signed-off-by: Alexander Stein <alexanders83@web.de>
Signed-off-by: David S. Miller <davem@davemloft.net>