[S390] cio: Use device_reprobe() instead of bus_rescan_devices().

In io_subchannel_register(), it is better to just reprobe the current
device if it hasn't a driver yet than to rescan the whole bus.

Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>