enic: fix last packet not being sent

The last packet of the tx burst function array was not being
emitted until the subsequent call.  The nic descriptor index
was being set to the current tx descriptor instead of one past
the descriptor as required by the nic.

Fixes: d739ba4c6abf ("enic: improve Tx packet rate")

Signed-off-by: John Daley <johndale@cisco.com>