The driver attempts to select an IRQ for the NIC automatically by
testing which of the supported IRQs are available and then probing
each available IRQ with probe_irq_{on,off}().  There are obvious race
conditions here, besides which:
1. The test for availability is done by passing a NULL handler, which
   now always returns -EINVAL, thus the device cannot be opened:
   <http://bugs.debian.org/566522>
2. probe_irq_off() will report only the first ISA IRQ handled,
   potentially leading to a false negative.

There was another bug that meant it ignored all error codes from
request_irq() except -EBUSY, so it would 'succeed' despite this
(possibly causing conflicts with other ISA devices).  This was fixed
by ab08999d 'WARNING: some
request_irq() failures ignored in el2_open()', which exposed bug 1.

This patch:
1. Replaces the use of probe_irq_{on,off}() with a real interrupt handler
2. Adds a delay before checking the interrupt-seen flag
3. Disables interrupts on all failure paths
4. Distinguishes error codes from the second request_irq() call,
   consistently with the first

Compile-tested only.
Signed-off-by: NBen Hutchings <ben@decadent.org.uk>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>