ehea: Dont check NETIF_F_TSO in TX path

It seems like the ehea xmit routine and an ethtool change of TSO
mode could race, resulting in corrupt packets. Checking gso_size
is enough and we can use the helper function.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>