pinctrl: pinmux: fix a possible null pointer in pinmux_can_be_used_for_gpio

pinctrl: pinmux: fix a possible null pointer in pinmux_can_be_used_for_gpio

This commit adds a check on ops pointer to avoid a kernel panic when
ops->strict is used. Indeed, on some pinctrl driver (at least for
pinctrl-stmfx) the pinmux ops is not implemented. Let's assume than gpio
can be used in this case.

Fixes: 472a61e777fe ("pinctrl/gpio: Take MUX usage into account")
Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>
Link: https://lore.kernel.org/r/20191204144106.10876-1-alexandre.torgue@st.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>