fm10k: Do not assume budget will never be 0 for NAPI

The netpoll path will call napi->poll with a budget of 0 in order to clean
the Tx rings only.  This change updates the fm10k driver so that it will
correctly support that instead of cleaning 1 Rx frame if a budget of 0 is
received.

Signed-off-by: Alexander Duyck <alexander.h.duyck@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>