cxgb4: Fix netdev_features flag

GRO is not supported by Chelsio HW when rx_csum is disabled.
Update the netdev features flag when rx_csum is modified.

Signed-off-by: Arjun Vynipadath <arjun@chelsio.com>
Signed-off-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Ganesh Goudar <ganeshgr@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>