s390/pci: remove bit_lock usage in interrupt handler

The interrupt handler uses bit_spin_lock around a call to retrieve
per irq data (the irq number). However this per irq data is only
set during irq setup time and never changed until the irq is freed.

Thus it's safe to remove the lock usage.

Signed-off-by: Sebastian Ott <sebott@linux.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>