Fix uart_set_ldisc() function type

Commit 64e9159f ("serial_core:
uart_set_ldisc infrastructure") introduced the ability for low-level
serial drivers to be informed when the tty ldisc changes.

However, the actual tty-layer function that does this callback for
serial devices was declared with the wrong type, having a spurious and
unused 'ldisc' argument.

This fixed the resulting compiler warning by just removing it.
Acked-by: Blithering Idiot <alan@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/serial/serial_core.c

@@ -1165,7 +1165,7 @@ out:
 	return ret;
 }
 
-static void uart_set_ldisc(struct tty_struct *tty, int ldisc)
+static void uart_set_ldisc(struct tty_struct *tty)
 {
 	struct uart_state *state = tty->driver_data;
 	struct uart_port *port = state->port;