natsemi: Fix NAPI for interrupt sharing

The interrupt status register for the natsemi chips is clear on read and
was read unconditionally from both the interrupt and from the NAPI poll
routine, meaning that if the interrupt service routine was called (for
example, due to a shared interrupt) while a NAPI poll was scheduled
interrupts could be missed.  This patch fixes that by ensuring that the
interrupt status register is only read by the interrupt handler when
interrupts are enabled from the chip.

It also reverts a workaround for this problem from the netpoll hook and
improves the trace for interrupt events.

Thanks to Sergei Shtylyov <sshtylyov@ru.mvista.com> for spotting the
issue, Mark Huth <mhuth@mvista.com> for a simpler method and Simon
Blake <simon@citylink.co.nz> for testing resources.

Signed-Off-By: Mark Brown <broonie@sirena.org.uk>

Signed-off-by: Jeff Garzik <jeff@garzik.org>