s390/ap: Fix bind complete udev event sent after each AP bus scan

With the mentioned commit (see the fixes tag) on every AP bus scan an
uevent "AP bus change bindings complete" is emitted.  Furthermore if an AP
device switched from one driver to another, for example by manipulating the
apmask, there was never a "bindings complete" uevent generated.

The "bindings complete" event should be sent once when all AP devices have
been bound to device drivers and again if unbind/bind actions take place
and finally all AP devices are bound again. Therefore implement this.

Fixes: 778412ab ("s390/ap: rearm APQNs bindings complete completion")
Reported-by: Marc Hartmayer <mhartmay@linux.ibm.com>
Signed-off-by: Harald Freudenberger <freude@linux.ibm.com>
Reviewed-by: Holger Dengler <dengler@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>