s390/scm_blk: consistently use blk_status_t as error type

Fix these warnings found by sparse:
drivers/s390/block/scm_blk.c:257:24: warning: incorrect type in assignment (different base types)
drivers/s390/block/scm_blk.c:257:24:    expected int [signed] <noident>
drivers/s390/block/scm_blk.c:257:24:    got restricted blk_status_t [usertype] error
drivers/s390/block/scm_blk.c:420:33: warning: incorrect type in argument 2 (different base types)
drivers/s390/block/scm_blk.c:420:33:    expected restricted blk_status_t [usertype] error
drivers/s390/block/scm_blk.c:420:33:    got int [signed] <noident>

Signed-off-by: Sebastian Ott <sebott@linux.vnet.ibm.com>
Reported-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>