libraries/SPI: abs -> std::abs and cast fixes (#7362)

* libraries/SPI: remove pointless abs(...) call

SPI library code erroneously assumed that:
- abs() is a C function, so include stdlib.h is required.
  what happens instead is Arduino.h shadows `abs()` with it's own macro
- uint32_t() - int32_t() promotes to int32_t, thus needing abs()

Fix both issues, leaving existing calculations as-is.

* additional changes for freq and constants

- restore abs call, cast freq to correctly display the intent
- update magic numbers comments
- move some spiclk_t magic numbers to func consts

libraries/SPI/SPI.h

@@ -22,7 +22,6 @@
 #define _SPI_H_INCLUDED
 
 #include <Arduino.h>
-#include <stdlib.h>
 
 #define SPI_HAS_TRANSACTION 1