dmaengine: PL08x: split DMA signal muxing from channel alloc

Split the DMA request mux signal handling from the physical channel
allocation code.  The physical channel has very little to do with the
DMA request input which will be used, so these should be two separate
operations.

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>