Fix using different SPI devices

[Yveaux]

Assignment of the SPI device to the SPIPORT reference doesn't seem to work on RP2040.
This fixes it by using a ptr.

[Yveaux]

Fixes #89

[michaelbeyhs]

Came here to make the same suggestion.
The begin() function takes an argument to specify the SPI device, but this doe not actually work.

[Yveaux]

This is still open, for a year now. Not much going on here unfortunately...