[S390] cio: change locking in io_subchannel_remove

IO subchannels are always unregistered in process context, so use
spin_lock_irq in the corresponding remove callback.

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