serial: drop debugging WARN_ON_ONCE() from uart_write()

syzbot is reporting lockdep warning upon

  int disc = 7;
  ioctl(open("/dev/ttyS3", O_RDONLY), TIOCSETD, &disc);

sequence. Do like what commit 5f1149d2 ("serial: drop debugging
WARN_ON_ONCE() from uart_put_char()") does.

Reported-by: syzbot+f78380e4eae53c64125c@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=f78380e4eae53c64125cSigned-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Acked-by: Jiri Slaby <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/d775ae2d-a2ac-439e-8e2b-134749f60f30@I-love.SAKURA.ne.jpSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/serial_core.c

@@ -622,7 +622,7 @@ static ssize_t uart_write(struct tty_struct *tty, const u8 *buf, size_t count)
 		return -EL3HLT;
 
 	port = uart_port_lock(state, flags);
-	if (WARN_ON_ONCE(!state->port.xmit_buf)) {
+	if (!state->port.xmit_buf) {
 		uart_port_unlock(port, flags);
 		return 0;
 	}