kw_gpio: fix error in kw_gpio_direction_input

The function kw_gpio_is_valid returns zero on success, so
adapt the error check accordingly.

Signed-off-by: Holger Brunck <holger.brunck@keymile.com>
cc: Prafulla Wadaskar <prafulla@marvell.com>
cc: Dieter Kiermaier <dk-arm-linux@gmx.de>