pinctrl/abx500: adjust offset for get_mode()

The set_mode() and get_mode() functions in the abx500 were
not mirrored, leading to the wrong GPIO control bits being
read out.

Cc: Lee Jones <lee.jones@linaro.org>
Reported-by: Patrice Chotard <patrice.chotard@st.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>