2. "irq ?" sets it to (u_short)-1
3. "irq #" sets it to (1<<#)
4. not specifying an interrupt sets it to 0.
Until someone else comes up with a better scheme, that's the way it is.
If you have a driver that turns the interrupt off, set it to ZERO.
If, after calling XXprobe(), id_irq is still (u_short)-1, that is the same
as if XXprobe() failed.