pinctrl: fix a minor harmless typo

The way the for_each_maps() macro is currently used, using "i" instead of
"_i_" works and is harmless. Still, this is a bug, that can trigger any
time, if the code around that macro changes. Better fix it now.

Acked-by: Stephen Warren <swarren@wwwdotorg.org>
Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>