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: NLinus Walleij <linus.walleij@linaro.org>
Tested-by: NLinus Walleij <linus.walleij@linaro.org>
Signed-off-by: NRussell King <rmk+kernel@arm.linux.org.uk>