pinctrl: stmfx: Use the callback to populate valid_mask

This makes use of the existing callback to populate the
valid mask instead of iteratively setting it up during
probe.

Cc: Benjamin Gaignard <benjamin.gaignard@st.com>
Cc: Amelie Delaunay <amelie.delaunay@st.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20190819091140.622-1-linus.walleij@linaro.org