dmaengine: PL08x: track mux usage on a per-channel basis.

Keep track of the number of descriptors currently using a MUX setting
on a per-channel basis.  This allows us to know when we have descriptors
queued somewhere which have been assigned a DMA request signal.

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>