gpio: Fix wrong rounding in gpio-menz127

men_z127_debounce() tries to round up and down, but uses functions which
are only suitable when the divider is a power of two, which is not the
case. Use the appropriate ones.

Found by static check. Compile tested.

Fixes: f436bc27

 ("gpio: add driver for MEN 16Z127 GPIO controller")
Signed-off-by: Nadav Amit <namit@vmware.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>