net: dsa: microchip: ksz8795: Don't use phy_port_cnt in VLAN table lookup

The magic number 4 in VLAN table lookup was the number of entries we
can read and write at once.  Using phy_port_cnt here doesn't make
sense and presumably broke VLAN filtering for 3-port switches.  Change
it back to 4.

Fixes: 4ce2a984 ("net: dsa: microchip: ksz8795: use phy_port_cnt ...")
Signed-off-by: Ben Hutchings <ben.hutchings@mind.be>
Signed-off-by: David S. Miller <davem@davemloft.net>