tty: serial: fsl_lpuart: flush receive FIFO after overruns

tty: serial: fsl_lpuart: flush receive FIFO after overruns

After overruns the FIFO pointers become misaligned. This
typically shows by characters still being stuck in the FIFO
despite the empty flag being asserted. After the first
assertion of the overrun flag the empty flag still seems to
indicate FIFO state correctly and all data can be read.
However, after another overrun assertion the FIFO seems to
be off by one such that the last received character is still
in the FIFO (despite the empty flag being asserted).

Flushing the receive FIFO reinitializes pointers. Hence it
is recommended to flush the FIFO after overruns, see also:
https://community.nxp.com/thread/321175

Hence, on assertion of the overrun flag read the remaining
data from the FIFO and flush buffers.

Signed-off-by: Stefan Agner <stefan.agner@toradex.com>
Acked-by: Max Krummenacher <max.krummenacher@toradex.com>
Cc: Stefan Agner <stefan@agner.ch>
Cc: Bhuvanchandra DV <bhuvanchandra.dv@toradex.com>
Cc: Chris Healy <cphealy@gmail.com>
Cc: Cory Tusar <cory.tusar@zii.aero>
Cc: Lucas Stach <l.stach@pengutronix.de>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jiri Slaby <jslaby@suse.com>
Cc: linux-imx@nxp.com
Cc: linux-serial@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Link: https://lore.kernel.org/r/20190729195226.8862-3-andrew.smirnov@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>