spi: rockchip: turn down tx dma bursts

This fixes tx and bi-directional dma transfers on rk3399-gru-kevin.

It seems the SPI fifo must have room for 2 bursts when the dma_tx_req
signal is generated or it might skip some words. This in turn makes
the rx dma channel never complete for bi-directional transfers.

Fix it by setting tx burst length to fifo_len / 4 and the dma
watermark to fifo_len / 2.

However the rk3399 TRM says (sic):
"DMAC support incrementing-address burst and fixed-address burst. But in
the case of access SPI and UART at byte or halfword size, DMAC only
support fixed-address burst and the address must be aligned to word."

So this relies on fifo_len being a multiple of 16 such that the
burst length (= fifo_len / 4) is a multiple of 4 and the addresses
will be word-aligned.

Fixes: dcfc861d

 ("spi: rockchip: adjust dma watermark and burstlen")
Signed-off-by: Emil Renner Berthing <kernel@esmil.dk>
Signed-off-by: Mark Brown <broonie@kernel.org>