spi: rockchip: set tx burst len 16, rx burst len 1.

spi: rockchip: set tx burst len 16, rx burst len 1.

Set ROCKCHIP_SPI_DMATDLR (rs->fifo_len - 16 - 1), which
can keep spi transferring. By the way, rx burst len must be
set 1, because it is hard to deal with the unaligned length.
Such as burst leng 16, ROCKCHIP_SPI_DMARDLR 16, when rx fifo
reaches 16, dma receive 16 bytes. But if the last bytes is less
than 16, dma will miss the bytes left in the rx fifo.

Change-Id: I846db94a87955453e617620ade32f2e68f01c01d
Signed-off-by: Huibin Hong <huibin.hong@rock-chips.com>