s390/cio: fix length calculation in idset.c

bitmap_or uses the number of bits as its length parameter and
not the number of words necessary to store those bits.

This fixes a regression introduced by:
	aa92b33c

 s390/cio: use generic bitmap functions

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