Fix SPI communication with overclocked Pico W

[maxgerhardt]

Just a basis for a possible fix for #2590.

Open points:

• instead of a simple div by 4 at a certain frequency, try to compute the (fractional) divider such that we always try to reach a certain SPI clock speed, such as 66.5MHz
• move fix to the lwip_cyw43439 library instead of the Pico W variant, to make it more generic? I wanted the call to be before the first cyw init call though

[earlephilhower]

Let's do this as part of #2616. I copied your changes here in addition to the refactor, so I think we're good. If not, we can reopen here and figure something out.