tty: serial: fsl_lpuart: fix lpuart32_poll_get_char

The watermark is set to 1, so we need to input two chars to trigger RDRF
using the original logic. With the new logic, we could always get the
char when there is data in FIFO.

Suggested-by: Fugang Duan <fugang.duan@nxp.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Link: https://lore.kernel.org/r/20200929095509.21680-1-peng.fan@nxp.com


Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>