The `PIO_NUM` macro was defined when `rp2_pio.c` was first conceived.
There's now a Pico SDK function for this, `pio_get_index()`, which is
already used in some parts of the code.
This commit removes `PIO_NUM` in favour of using `pio_get_index()`
everywhere.
Signed-off-by: Phil Howard <github@gadgetoid.com>
462fa5f94f