tty: serial: fsl_lpuart: fix framing error handling when using DMA

When using DMA framing error get cleared properly. However, due
to the additional read from the data register, an underflow in
the receive FIFO buffer occurs (the FIFO pointer gets out of
sync).

Clear the FIFO in case an underflow has occurred. Also disable the
receiver during this operation and when reading the data register to
minimize potential interference.

Signed-off-by: Stefan Agner <[email protected]>
Acked-by: Max Krummenacher <[email protected]>
Signed-off-by: Andrey Smirnov <[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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