uli526x: add support for netpoll

This patch adds netpoll support for the uli526x ethernet driver --
simply call the interrupt handler for polling.

To do this without disable_irq()/enable_irq() pair we should fully
protect the handler. Luckily, it's already using irqsave spinlock,
the only unprotected place is interrupts re-enabling write. It was
safe to re-enable interrupts without holding the spinlock, but with
netpoll possibility now it doesn't seem so.

Patch was tested using netconsole and KGDBoE.

Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>