[S390] irq: fix accounting of external call/emergency signal

Mask the extint_code parameter of the smp external interrupt handler
to get the interruption code. Otherwise emergency call interrupts
erroneously might be accounted as emergency signal interrupts.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>