spi/topcliff-pch: Fix DMA channel

bus_num might be asigned dynamically to e.g. 32766. In this case the
calculated DMA channel based on SPI bus number is bogus. Use SPI channel
number instead for calculation.

Signed-off-by: Alexander Stein <alexander.stein@systec-electronic.com>
Signed-off-by: Mark Brown <broonie@linaro.org>