drivers: gpio: fix misuse of u8_t where bool is intended

GPIO configuration flags will move and some that used to be in the low
8 bits are now higher, resulting in implicit constant conversion
overflows.  Use a boolean data type to hold boolean values.

Signed-off-by: Peter A. Bigot <pab@pabigot.com>
Signed-off-by: Piotr Mienkowski <piotr.mienkowski@gmail.com>