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 5f1149d2f4bf ("serial: drop debugging
WARN_ON_ONCE() from uart_put_char()") does.

Reported-by: [e-mail]
Closes: https://syzkaller.appspot.com/bug?extid=f78380e4eae53c64125c
Signed-off-by: Tetsuo Handa <[e-mail]>
Acked-by: Jiri Slaby <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/tty/serial/serial_core.c

···
 		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;
 	}