dmaengine/amba-pl08x: Call pl08x_free_txd() instead of calling kfree() directly

pl08x_prep_channel_resources() is calling kfree() directly for txd(). To
maintain consistency in code call pl08x_free_txd() instead.

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>