[S390] cio: online_store - trigger recognition for boxed devices

Start a new device recognition if someone writes to sysfs online attribute
of a boxed ccw device. The current test will fail, since cu_type != 0
for devices which were recognized before.

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