dmaengine: PL08x: re-jig the starting of txds

Rather than code the de-queue of the txd several times, move that into
the start_txd function.  Rename this to better illustrate what it's
now doing, and call this function when starting a delayed memcpy().

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>