tty: make receive_buf() return the amout of bytes received

it makes it simpler to keep track of the amount of
bytes received and simplifies how flush_to_ldisc counts
the remaining bytes. It also fixes a bug of lost bytes
on n_tty when flushing too many bytes via the USB
serial gadget driver.

Tested-by: Stefan Bigler <stefan.bigler@keymile.com>
Tested-by: Toby Gray <toby.gray@realvnc.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
diff --git a/drivers/net/hamradio/6pack.c b/drivers/net/hamradio/6pack.c
index 3e5d0b6..9920896 100644
--- a/drivers/net/hamradio/6pack.c
+++ b/drivers/net/hamradio/6pack.c
@@ -456,7 +456,7 @@
  * a block of 6pack data has been received, which can now be decapsulated
  * and sent on to some IP layer for further processing.
  */
-static void sixpack_receive_buf(struct tty_struct *tty,
+static unsigned int sixpack_receive_buf(struct tty_struct *tty,
 	const unsigned char *cp, char *fp, int count)
 {
 	struct sixpack *sp;
@@ -464,11 +464,11 @@
 	int count1;
 
 	if (!count)
-		return;
+		return 0;
 
 	sp = sp_get(tty);
 	if (!sp)
-		return;
+		return -ENODEV;
 
 	memcpy(buf, cp, count < sizeof(buf) ? count : sizeof(buf));
 
@@ -487,6 +487,8 @@
 
 	sp_put(sp);
 	tty_unthrottle(tty);
+
+	return count1;
 }
 
 /*