net: fec: iMX6 FEC does not support half-duplex gigabit

The iMX6 gigabit FEC does not support half-duplex gigabit operation.
Phys attacked to the FEC may support this, and we currently do nothing
to disable this feature.  This may result in an invalid configuration.
Mask out phy support for gigabit half-duplex operation.

Acked-by: Fugang Duan <B38611@freescale.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>