enic: fix rx napi poll return value

With the commit d75b1ade ("net: less interrupt masking in NAPI") napi repoll
is done only when work_done == budget. When we are in busy_poll we return 0 in
napi_poll. We should return budget.

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