be2net: fix spurious interrupt handling in intx mode

Occasionally we may see an interrupt without an event in the eq.
In intx, we currently see the event queue and return IRQ_NONE causing
a the irq to be disabled ("no one cared".) Instead, read the CEV_ISR
reg to check the existence of the interrupt.

Signed-off-by: Sathya Perla <sathyap@serverengines.com>
Signed-off-by: David S. Miller <davem@davemloft.net>