gpio: of: Fix NPE from OF flags

Some calls to of_get_named_gpio() calls sets the flags
argument to NULL because they are not interested in the
flags. This caused a null pointer exception since we were
unconditionally using these flags. Fix it.

Fixes: 6a537d48 ("gpio: of: Support regulator nonstandard GPIO properties")
Cc: Heiner Kallweit <hkallweit1@gmail.com>
Reported-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>