enic: fix Tx hang when number of packets > queue size

enic: fix Tx hang when number of packets > queue size

If the nb_pkts parameter to rte_eth_tx_burst() was greater than
the TX descriptor count, a completion was not being requested
from the NIC, so descriptors would not be released back to the
host causing a lock-up.

Introduce a limit of how many TX descriptors can be used in a single
call to the enic PMD burst TX function before requesting a completion.

Fixes: d739ba4c6abf ("enic: improve Tx packet rate")
Signed-off-by: John Daley <johndale@cisco.com>