serial: fsl_lpuart: use UARTDATA_MASK macro

Use the corresponding macro instead of the magic number. While at it,
drop the useless cast to "unsigned char".

Signed-off-by: Michael Walle <michael@walle.cc>
Link: https://lore.kernel.org/r/20210512141255.18277-3-michael@walle.cc


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>