media: smsusb: better handle optional alignment

Most Siano devices require an alignment for the response.

Changeset f3be52b0056a ("media: usb: siano: Fix general protection fault in smsusb")
changed the logic with gets such aligment, but it now produces a
sparce warning:

drivers/media/usb/siano/smsusb.c: In function 'smsusb_init_device':
drivers/media/usb/siano/smsusb.c:447:37: warning: 'in_maxp' may be used uninitialized in this function [-Wmaybe-uninitialized]
  447 |   dev->response_alignment = in_maxp - sizeof(struct sms_msg_hdr);
      |                             ~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~

The sparse message itself is bogus, but a broken (or fake) USB
eeprom could produce a negative value for response_alignment.

So, change the code in order to check if the result is not
negative.

Fixes: 31e0456d ("media: usb: siano: Fix general protection fault in smsusb")
CC: <stable@vger.kernel.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/media/usb/siano/smsusb.c

@@ -400,7 +400,7 @@ static int smsusb_init_device(struct usb_interface *intf, int board_id)
 	struct smsusb_device_t *dev;
 	void *mdev;
 	int i, rc;
-	int in_maxp = 0;
+	int align = 0;
 
 	/* create device object */
 	dev = kzalloc(sizeof(struct smsusb_device_t), GFP_KERNEL);
@@ -418,14 +418,14 @@ static int smsusb_init_device(struct usb_interface *intf, int board_id)
 
 		if (desc->bEndpointAddress & USB_DIR_IN) {
 			dev->in_ep = desc->bEndpointAddress;
-			in_maxp = usb_endpoint_maxp(desc);
+			align = usb_endpoint_maxp(desc) - sizeof(struct sms_msg_hdr);
 		} else {
 			dev->out_ep = desc->bEndpointAddress;
 		}
 	}
 
 	pr_debug("in_ep = %02x, out_ep = %02x\n", dev->in_ep, dev->out_ep);
-	if (!dev->in_ep || !dev->out_ep) {	/* Missing endpoints? */
+	if (!dev->in_ep || !dev->out_ep || align < 0) {  /* Missing endpoints? */
 		smsusb_term_device(intf);
 		return -ENODEV;
 	}
@@ -444,7 +444,7 @@ static int smsusb_init_device(struct usb_interface *intf, int board_id)
 		/* fall-thru */
 	default:
 		dev->buffer_size = USB2_BUFFER_SIZE;
-		dev->response_alignment = in_maxp - sizeof(struct sms_msg_hdr);
+		dev->response_alignment = align;
 
 		params.flags |= SMS_DEVICE_FAMILY2;
 		break;