spi: spi-fsl-lpspi: fsl_lpspi_set_watermark(): use FIELD_PREP() to encode FIFO Contro...

spi: spi-fsl-lpspi: fsl_lpspi_set_watermark(): use FIELD_PREP() to encode FIFO Control register

Instead of open coding mask and shift operations and to increase
readability use FIELD_PREP() to encode the FIFO Control register.

Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
Link: https://patch.msgid.link/20260319-spi-fsl-lpspi-cleanups-v2-2-02b56c5d44a8@pengutronix.de
Signed-off-by: Mark Brown <broonie@kernel.org>