enic: fix work done in tx napi_poll

With the commit d75b1ade ("net: less interrupt masking in NAPI") napi repoll
is done only when work_done == budget. In tx napi poll we always return 0.
So tx napi is not called again and we do not clean up the tx ring.

Signed-off-by: Govindarajulu Varadarajan <_govind@gmx.com>
Signed-off-by: David S. Miller <davem@davemloft.net>