pinctrl: abx500: Fix checking if pin use AlternateFunction register

It's pointless to check "af.alt_bit1 == UNUSED" twice.
This looks like a copy-paste bug, I think what we want is to check if *both*
af.alt_bit1 and af.alt_bit2 are UNUSED.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Patrice Chotard <patrice.chotard@st.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>