[S390] s390mach compile warning

Fix the following compile warning:

drivers/s390/s390mach.c: In function 's390_collect_crw_info':
drivers/s390/s390mach.c:77: warning: ignoring return value of 'down_interruptibl

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>