s390/qeth: don't warn for napi with 0 budget

Calling napi->poll() with 0 budget is a legitimate use by netpoll.

Fixes: a1c3ed4c

 ("qeth: NAPI support for l2 and l3 discipline")
Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>