dmaengine: PL08x: get rid of unnecessary checks in dma_slave_config

Get rid of the unnecessary checks in dma_slave_config utilizing
the DMA direction.  This allows us to move the computation of
cctl to the prepare function.

Acked-by: Linus Walleij <linus.walleij@linaro.org>
Tested-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>