s390/cio: use generic bitmap functions

Use generic bitmap functions in the subchannel id bitmap to
simplify and de-bloat the code.

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