[S390] sysinfo: fix SYSIB 3,2,2 structure

The size of the field that contains the description block count is
only four bits instead of eight bits.
The first four bits are reserved but this might change and break.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>