scsi: st: Fix fall-through warning for Clang

Fix the following fallthrough warning:

drivers/scsi/st.c:3826:2: error: unannotated fall-through between switch labels [-Werror,-Wimplicit-fallthrough]
drivers/scsi/st.c:3826:2: note: insert 'break;' to avoid fall-through

This helps with the ongoing efforts to globally enable
-Wimplicit-fallthrough for Clang.

Link: https://github.com/KSPP/linux/issues/115
Link: https://lore.kernel.org/linux-mm/202109070707.4pPukuYB-lkp@intel.com/
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>

drivers/scsi/st.c

@@ -3823,6 +3823,7 @@ static long st_ioctl(struct file *file, unsigned int cmd_in, unsigned long arg)
 	case CDROM_SEND_PACKET:
 		if (!capable(CAP_SYS_RAWIO))
 			return -EPERM;
+		break;
 	default:
 		break;
 	}