s390/cio: generate delayed uevent for vfio-ccw subchannels

The common I/O layer delays the ADD uevent for subchannels and
delegates generating this uevent to the individual subchannel
drivers. The vfio-ccw I/O subchannel driver, however, did not
do that, and will not generate an ADD uevent for subchannels
that had not been bound to a different driver (or none at all,
which also triggers the uevent).

Generate the ADD uevent at the end of the probe function if
uevents were still suppressed for the device.

Message-Id: <20200327124503.9794-3-cohuck@redhat.com>
Fixes: 63f1934d ("vfio: ccw: basic implementation for vfio_ccw driver")
Reviewed-by: Eric Farman <farman@linux.ibm.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>