s390: pci: Check return value of alloc_irq_desc() proper

s390: pci: Check return value of alloc_irq_desc() proper

alloc_irq_desc() returns an integer and as documented either a valid
irq number or a negative error code. Checking for NO_IRQ is definitely
not the proper error handling.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Grant Likely <grant.likely@linaro.org>
Cc: Tony Luck <tony.luck@intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: linux390@de.ibm.com
Link: http://lkml.kernel.org/r/20140507154339.409085048@linutronix.de
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>