net: dsa: mv88e6xxx: use BMCR definitions for serdes control register

The SGMII/1000base-X serdes register set is a clause 22 register set
offset at 0x2000 in the PHYXS device. Rather than inventing our own
defintions, use those that already exist, and name the register
MV88E6390_SGMII_BMCR.  Also remove the unused MV88E6390_SGMII_STATUS
definitions.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>