net: dsa: mv88e6xxx: use generic clause 45 definitions

The private MV88E6390_PCS_CONTROL_1 definitions in serdes.h reflects
the IEEE 802.3 standard PCS control register 1 definitions, only
offset by 0x1000 in the PHYXS register space.  Rather than inventing
our own, use those that already exist, and name the register
MV88E6390_10G_CTRL1.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>