serial: imx: get rid of unbounded busy-waiting loop

imx_set_termios(): remove busy-waiting "drain Tx FIFO" loop. Worse
yet, it was potentially unbounded wait due to RTS/CTS (hardware)
handshake.

Let user space ensure draining is done before termios change, if
draining is needed in the first place.

Signed-off-by: Sergey Organov <sorganov@gmail.com>
Link: https://lore.kernel.org/r/1567017475-11919-2-git-send-email-sorganov@gmail.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>