scsi: scsi_debug: Fix an error handling bug in sdeb_zbc_model_str()

This test is checking the wrong variable.  It should be testing "res".
The "sdeb_zbc_model" variable is an enum (unsigned in this situation)
and we never assign negative values to it.

[mkp: fixed commit desc issue reported by Doug]

Link: https://lore.kernel.org/r/20200509100408.GA5555@mwanda
Fixes: 9267e0eb ("scsi: scsi_debug: Add ZBC module parameter")
Acked-by: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

drivers/scsi/scsi_debug.c

@@ -6460,7 +6460,7 @@ static int sdeb_zbc_model_str(const char *cp)
 		res = sysfs_match_string(zbc_model_strs_b, cp);
 		if (res < 0) {
 			res = sysfs_match_string(zbc_model_strs_c, cp);
-			if (sdeb_zbc_model < 0)
+			if (res < 0)
 				return -EINVAL;
 		}
 	}