Refactor gpiochip_allocate_mask() slightly by using bitmap_alloc().

I used bitmap_free() for the corresponding free parts. Actually,
bitmap_free() is a wrapper of kfree(), but I did this for consistency.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Link: https://lore.kernel.org/r/20190718065101.26994-1-yamada.masahiro@socionext.comReviewed-by: Stephen Boyd <swboyd@chromium.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>