serial: core: Fix port mutex assert if lockdep disabled

commit 4047b37122 ("serial: core: Prevent unsafe uart port access,
part 1") added lockdep assertion for port mutex but fails to check
if debug_locks has disabled lockdep (so lock state is no longer
reliable).

Use lockdep_assert_held() instead, which properly checks lockdep
state as well.

Reported-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Tested-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/serial_core.c

@@ -95,9 +95,7 @@ static inline void uart_port_deref(struct uart_port *uport)
 
 static inline struct uart_port *uart_port_check(struct uart_state *state)
 {
-#ifdef CONFIG_LOCKDEP
-	WARN_ON(!lockdep_is_held(&state->port.mutex));
-#endif
+	lockdep_assert_held(&state->port.mutex);
 	return state->uart_port;
 }