gpio-sta2x11: don't use pdata if null

If there is no platform data available, the driver shouldn't use the
pointer or it will oops.  Since things will mostly work nonetheless,
(the BIOS may have set up the pins properly), I'd better not fail the
probe even in this case.

Signed-off-by: Alessandro Rubini <rubini@gnudd.com>
Acked-by: Giancarlo Asnaghi <giancarlo.asnaghi@st.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>