dmaengine: PL08x: remove waiting descriptor pointer

As we no longer need to pass a descriptor to prep_phy_channel(), we
don't need to keep track of the descriptor which is waiting for a
channel to become available.  So let's get rid of it.

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>