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 <[email protected]>
Acked-by: Max Krummenacher <[email protected]>
Cc: Stefan Agner <[email protected]>
Cc: Bhuvanchandra DV <[email protected]>
Cc: Chris Healy <[email protected]>
Cc: Cory Tusar <[email protected]>
Cc: Lucas Stach <[email protected]>
Cc: Greg Kroah-Hartman <[email protected]>
Cc: Jiri Slaby <[email protected]>
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>

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