serial: fsl_lpuart: clear parity enable bit when disable parity

Current driver only enable parity enable bit and never clear it
when user set the termios. The fix clear the parity enable bit when
PARENB flag is not set in termios->c_cflag.

Cc: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Andy Duan <fugang.duan@nxp.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>