tty: serial: fsl_lpuart: fix del_timer_sync() vs timer routine deadlock

Problem found via lockdep:

- lpuart_set_termios() calls del_timer_sync(&sport->lpuart_timer) while
  holding sport->port.lock

- sport->lpuart_timer routine is lpuart_timer_func() that calls
  lpuart_copy_rx_to_tty() that acquires same lock.

To fix, move Rx DMA stopping out of lock, as it already is in other places
in the same file.

While at it, also make Rx DMA start/stop code to look the same is in
other places in the same file.

Signed-off-by: Nikita Yushchenko <nikita.yoush@cogentembedded.com>
Tested-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>