ionic: Shorten a Tx hotpath

Perf was showing some hot spots in ionic_tx_descs_needed()
for TSO traffic.  Rework the function to return sooner where
possible.

Signed-off-by: Brett Creeley <brett.creeley@amd.com>
Signed-off-by: Shannon Nelson <shannon.nelson@amd.com>
Signed-off-by: David S. Miller <davem@davemloft.net>