gpiolib: Fix missing updates of bitmap index

In new code introduced by commit b17566a6 ("gpiolib: Implement fast
processing path in get/set array"), bitmap index is not updated with
next found zero bit position as it should while skipping over pins
already processed via fast bitmap path, possibly resulting in an
infinite loop.  Fix it.

Signed-off-by: Janusz Krzysztofik <jmkrzyszt@gmail.com>
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>