spi: rockchip: Wait for STB status in slave mode tx_xfer

After ROCKCHIP_SPI_VER2_TYPE2, SR->STB is a more accurate judgment
bit for spi slave transmition.

Signed-off-by: Jon Lin <jon.lin@rock-chips.com>
Link: https://lore.kernel.org/r/20210621104800.19088-5-jon.lin@rock-chips.com


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