scsi: tcmu: Don't pass KERN_ERR to pr_err

Fix warning:

smatch warnings:
drivers/target/target_core_user.c:301 tcmu_genl_cmd_done() warn: KERN_*
level not at start of string

Signed-off-by: Mike Christie <mchristi@redhat.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>