s390x/css: Fix css stsch detection to go beyond 8192 devices

find_last_bit expects a size in bits and not in words. the calculation
limits the number of guest devices to 8192.  This probably does not
matter in reality, its wrong anyway.

Fixes: df1fe5bb49241 ("s390: Virtual channel subsystem support")
Signed-off-by: Christian Borntraeger <borntraeger@linux.ibm.com>
Reviewed-by: Matthew Rosato <mjrosato@linux.ibm.com>
Reviewed-by: Eric Farman <farman@linux.ibm.com>
Link: https://lore.kernel.org/qemu-devel/20260814152226.1834819-1-borntraeger@de.ibm.com
Signed-off-by: Eric Farman <farman@linux.ibm.com>