ionic: generic tx skb mapping

Make the new ionic_tx_map_tso() usable by the non-TSO paths,
and pull the call up a level into ionic_tx() before calling
the csum or no-csum routines.

Signed-off-by: Shannon Nelson <snelson@pensando.io>
Signed-off-by: David S. Miller <davem@davemloft.net>