media: flexcop-usb: ensure -EIO is returned on error condition

An earlier commit hard coded a return 0 to function flexcop_usb_i2c_req
even though the an -EIO was intended to be returned in the case where
ret != buflen.  Fix this by replacing the return 0 with the return of
ret to return the error return code.

Addresses-Coverity: ("Unused value")

Fixes: b430eaba0b ("[media] flexcop-usb: don't use stack for DMA")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>

drivers/media/usb/b2c2/flexcop-usb.c

@@ -295,7 +295,7 @@ static int flexcop_usb_i2c_req(struct flexcop_i2c_adapter *i2c,
 
 	mutex_unlock(&fc_usb->data_mutex);
 
-	return 0;
+	return ret;
 }
 
 /* actual bus specific access functions,