spi: spi-sun6i: sun6i_spi_get_tx_fifo_count: Convert manual shift+mask to FIELD_GET()

This patch converts the manual shift+mask in sun6i_spi_get_tx_fifo_count() to
make use of FIELD_GET()

Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
Link: https://lore.kernel.org/r/20200706143443.9855-6-mkl@pengutronix.de


Signed-off-by: Mark Brown <broonie@kernel.org>