dmaengine/amba-pl08x: No need to check "ch->signal < 0"

We have just executed following in pl08x_get_phy_channel():
	ch->signal = -1;

We don't have to compare "ch->signal < 0", as this will always be true.

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>