dmaengine: PL08x: fix tx_status function to return correct residue

Now that we're converted to use the generic vchan support, we can fix
the residue return from tx_status to be compliant with dmaengine.  This
returns the number of bytes remaining for the _specified_ cookie, not
the number of bytes in all pending transfers on the channel.

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>