prevent sending data when CTS is de-asserted.
In uart_tty_intr(), call uart_tty_outwakeup() when the CTS signal
changed, knowing that uart_tty_outwakeup() will do the right
thing for flow control. This avoids redundant conditionals.
PR: kern/148644
Submitted by: John Wehle <john@feith.com>
MFC after: 3 days