pinctrl: pinconf: take the right mutex

The pinconf_dgb_config_print() takes the per-pincontroller
mutex, when what it wants to take is actually the pin maps
mutex.

Reported-by: James Hogan <james.hogan@imgtec.com>
Cc: Patrice Chotard <patrice.chotard@st.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>