net: dsa: mv88e6xxx: Fix mv88e6393x EPC write command offset

According to datasheet, the command opcode must be specified
into bits [14:12] of the Extended Port Control register (EPC).

Fixes: de776d0d ("net: dsa: mv88e6xxx: add support for mv88e6393x family")
Signed-off-by: Marco Migliore <m.migliore@tiesse.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>