dmaengine: PL08x: get rid of device_fc in struct pl08x_dma_chan

As we now store the dma_slave_config in pl08x_dma_chan, we don't need
to store this separately.  Use the one in dma_slave_config directly.

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>