pinctrl: stmfx: fix null pointer on remove

dev_get_platdata(&pdev->dev) returns a pointer on struct stmfx_pinctrl,
not on struct stmfx (platform_set_drvdata(pdev, pctl); in probe).
Pointer on struct stmfx is stored in driver data of pdev parent (in probe:
struct stmfx *stmfx = dev_get_drvdata(pdev->dev.parent);).

Fixes: 1490d9f8

 ("pinctrl: Add STMFX GPIO expander Pinctrl/GPIO driver")
Signed-off-by: Amelie Delaunay <amelie.delaunay@st.com>
Link: https://lore.kernel.org/r/20191004122342.22018-1-amelie.delaunay@st.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>