3c503: Fix IRQ probing

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 ab08999d6029bb2c79c16be5405d63d2bedbdfea '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: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>