staging: et131x: Remove private adapter->duplex_mode and use phydev->duplex instead

The phy device keeps a note of the duplex mode, so use that value instead of duplex mode.

Also use the phydev defines for duplex modes, and remove the driver private ones.

Signed-off-by: Mark Einon <mark.einon@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>