be2net: fix unconditionally returning IRQ_HANDLED in INTx

commit e49cc34f introduced an unconditional IRQ_HANDLED return in be_intx()
to workaround Lancer and BE2 HW issues. This is bad as it prevents the kernel
from detecting interrupt storms due to broken HW.

The BE2/Lancer HW issues are:
1) In Lancer, there is no means for the driver to detect if the interrupt
belonged to device, other than counting and notifying events.
2) In Lancer de-asserting INTx takes a while, causing the INTx irq handler
to be called multiple times till the de-assert happens.
3) In BE2, we see an occasional interrupt even when EQs are unarmed.

Issue (1) can cause the notified events to be orphaned, if NAPI was already
running.
This patch fixes this issue by scheduling NAPI only if it is not scheduled
already. Doing this also takes care of possible events_get() race that may be
caused due to issue (2) and (3). Also, IRQ_HANDLED is returned only the first
time zero events are detected.
(Thanks Ben H. for the feedback and suggestions.)

Signed-off-by: Sathya Perla <sathya.perla@emulex.com>
Signed-off-by: David S. Miller <davem@davemloft.net>