net: dsa: lan9303: Adjust phy_addr_base expressions

Simplify calculation of chip->phy_addr_base in lan9303_detect_phy_setup().

Use GENMASK to calculate phys_mii_mask from LAN9303_NUM_PORTS and
phy_addr_base.

Signed-off-by: Egil Hjelmeland <privat@egil-hjelmeland.no>
Signed-off-by: David S. Miller <davem@davemloft.net>