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/caif/caif_serial.c b/drivers/net/caif/caif_serial.c
index 3df0c0f..73c7e03 100644
--- a/drivers/net/caif/caif_serial.c
+++ b/drivers/net/caif/caif_serial.c
@@ -167,8 +167,8 @@
 
 #endif
 
-static void ldisc_receive(struct tty_struct *tty, const u8 *data,
-			char *flags, int count)
+static unsigned int ldisc_receive(struct tty_struct *tty,
+		const u8 *data, char *flags, int count)
 {
 	struct sk_buff *skb = NULL;
 	struct ser_device *ser;
@@ -215,6 +215,8 @@
 	} else
 		++ser->dev->stats.rx_dropped;
 	update_tty_status(ser);
+
+	return count;
 }
 
 static int handle_tx(struct ser_device *ser)