[SCSI] qla2xxx: Handle IRQ-0 assignments by the system.

No restriction should be placed on the IRQ number assigned
to a given ISP.  Original code incorrectly assumed a
non-zero IRQ number assignment by the system.  In these
circumstances the proper freeing of the IRQ (via free_irq())
would not take place.

Signed-off-by: Andrew Vasquez <andrew.vasquez@qlogic.com>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>