natsemi: netpoll fixes

Fix two issues in this driver's netpoll path: one usual, with spin_unlock_irq()
enabling interrupts which nobody asks it to do (that has been fixed recently in
a number of drivers) and one unusual, with poll_controller() method possibly
causing loss of interrupts due to the interrupt status register being cleared
by a simple read and the interrpupt handler simply storing it, not accumulating.

Signed-off-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>