enetc: Allow to disable Tx SG

The fact that the Tx SG flag is fixed to 'on' is only
an oversight. Non-SG mode is also supported. Fix this
by allowing to turn SG off.

Signed-off-by: Claudiu Manoil <claudiu.manoil@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>