dmaengine: PL08x: remove circular_buffer boolean from channel data

Circular buffers are not handled in this way; we have a separate API
call now to setup circular buffers.  So lets not mislead people with
this bool.

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>