pinctrl: st: Remove unnecessary use of of_match_ptr macro

This is a DT only driver and st_pctl_of_match is always compiled
in. Hence of_match_ptr is unnecessary.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Mark Brown <broonie@linaro.org>