pinctrl: sunxi: Fix and simplify pin bank regulator handling

The new per-pin-bank regulator handling code in the sunxi pinctrl driver
has mismatched conditions for enabling and disabling the regulator: it
is enabled each time a pin is requested, but only disabled when the
pin-bank's reference count reaches zero.

Since we are doing reference counting already, there's no need to enable
the regulator each time a pin is requested. Instead we can just do it
for the first requested pin of each pin-bank. Thus we can reverse the
test and bail out early if it's not the first occurrence.

Fixes: 9a2a566a ("pinctrl: sunxi: Deal with per-bank regulators")
Acked-by: Maxime Ripard <maxime.ripard@bootlin.com>
Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>