s390x/css: {c,h,t,r,x}sch: require enable AND device number valid

According to the PoP, subchannels are only considered operational if
they are enabled _and_ the device number is valid. With the current
checks being enabled _or_ having a valid device number was
sufficient. This caused qemu to allow IO on subchannels that were not
enabled.

Fix the checks to require both bits to be set.

Signed-off-by: Sascha Silbe <silbe@linux.vnet.ibm.com>
Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Reviewed-by: Halil Pasic <pasic@linux.vnet.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
