spi: rockchip: Config spi rx dma burst size depend on xfer length

The burst length can be adjusted according to the transmission
length to improve the transmission rate

Signed-off-by: Jon Lin <jon.lin@rock-chips.com>
Tested-by: Emil Renner Berthing <kernel@esmil.dk>
Reviewed-by: Emil Renner Berthing <kernel@esmil.dk>
Link: https://lore.kernel.org/r/20200723004356.6390-1-jon.lin@rock-chips.com


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