gpio: of: Handle fixed regulator flags properly

This fixes up the handling of fixed regulator polarity
inversion flags: while I remembered to fix it for the
undocumented "reg-fixed-voltage" I forgot about the
official "regulator-fixed" binding, there are two ways
to do a fixed regulator.

The error was noticed and fixed.

Fixes: a603a2b8 ("gpio: of: Add special quirk to parse regulator flags")
Cc: Mark Brown <broonie@kernel.org>
Cc: Thierry Reding <thierry.reding@gmail.com>
Reported-by: Thierry Reding <thierry.reding@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>