s390/con3270: return from notifier when activate view fails

When activating the view fails (in this case because the 3270
is disconnected) return from the notifer callback. Otherwise
the system will deadlock.

Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Acked-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>