net: dsa: mv88e6xxx: kill last locked reg_read

Get rid of the last usage of the locked mv88e6xxx_reg_read function with
a new mv88e6xxx_port_read helper, useful later for chips with different
port registers base address.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>