tty: n_gsm: Don't ignore write return value in gsmld_output()

We currently have gsmld_output() ignore the return value from device
write. This means we will lose packets if device write returns 0 or
an error.
Signed-off-by: NTony Lindgren <tony@atomide.com>
Link: https://lore.kernel.org/r/20210930060624.46523-1-tony@atomide.comSigned-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/n_gsm.c

@@ -695,7 +695,7 @@ static void gsm_data_kick(struct gsm_mux *gsm, struct gsm_dlci *dlci)
 			print_hex_dump_bytes("gsm_data_kick: ",
 					     DUMP_PREFIX_OFFSET,
 					     gsm->txframe, len);
-		if (gsmld_output(gsm, gsm->txframe, len) < 0)
+		if (gsmld_output(gsm, gsm->txframe, len) <= 0)
 			break;
 		/* FIXME: Can eliminate one SOF in many more cases */
 		gsm->tx_bytes -= msg->len;
@@ -2415,8 +2415,7 @@ static int gsmld_output(struct gsm_mux *gsm, u8 *data, int len)
 	if (debug & 4)
 		print_hex_dump_bytes("gsmld_output: ", DUMP_PREFIX_OFFSET,
 				     data, len);
-	gsm->tty->ops->write(gsm->tty, data, len);
-	return len;
+	return gsm->tty->ops->write(gsm->tty, data, len);
 }
 
 /**