dmaengine: PL08x: ensure all descriptors are freed when channel is released

Ensure all queued descriptors are freed when the channel is released,
ensuring we don't leak memory

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>