drivers: mcux_lpuart: clear Received Overrun Flag

According to i.MX RT1060 Reference Manual:

  While the OR flag is set, no additional data is stored in the data
  buffer even if sufficient room exists. To clear OR, write logic 1 to
  the OR flag.

Clear OR (Overrun) flag whenever it is set, so that data continues to be
received after potential data overrun.

Signed-off-by: Marcin Niestroj <m.niestroj@emb.dev>