drivers: gpio_rpi_pico: Disable inputs when not in use

Reorder gpio_rpi_configure to disable input buffers when not in use.
gpio_rpi_get_config can then determine whether a pin is configured as an
input without requiring additional state variables, as well as reducing
input leakage current.

Signed-off-by: Andrew Featherstone <andrew.featherstone@gmail.com>