spi: spi-sun6i: sun6i_spi_drain_fifo(): introduce sun6i_spi_get_rx_fifo_count() and make use of it

This patch introduces the function sun6i_spi_get_rx_fifo_count(), similar to
the existing sun6i_spi_get_tx_fifo_count(), to make the sun6i_spi_drain_fifo()
function a bit easier to read.

Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
Acked-by: Maxime Ripard <mripard@kernel.org>
Link: https://lore.kernel.org/r/20200706143443.9855-7-mkl@pengutronix.de


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