drivers: misc: pio_rpi_pico: Make `pio_rpi_pico_get_pio` inline.

The `pio` value is stored in the internal `pio_rpi_pico_config` structure
that is not exposed. However, it is the first element of the structure.
Here, I convert the pointer to `pio_rpi_pico_config` into a pointer to PIO
to access this value.

Signed-off-by: Terry Geng <terry@terriex.com>