gpio: adp5588 - use "unsigned" for the setup and teardown callbacks

to comply with the rest of the GPIO drivers.

Signed-off-by: Jean-Francois Dagenais <jeff.dagenais@gmail.com>
Acked-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>