dmaengine: PL08x: clean up get_signal/put_signal

Try to avoid dereferencing the DMA engine's channel struct in these
platform helpers; instead, pass a pointer to the channel data into
get_signal(), and the returned signal number to put_signal().

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