dmaengine/amba-pl08x: Complete doc comment for struct pl08x_txd

Doc comment for struct pl08x_txd was incomplete. Complete that.

Signed-off-by: Viresh Kumar <viresh.kumar@st.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>