net: dsa: mv88e6xxx: export mv88e6xxx_pcs_decode_state()

Rename and export the PCS state decoding function so our PCS can
make use of the functionality provided by this.

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