libraries/SPI: abs -> std::abs and cast fixes

[mcspr]

Not to complicate comment chain in #6294 further

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
freq - calFreq or freq - bestFreq may produce negative results and require abs()
In the referenced PR, when adding using std::abs; it confuses GCC by introducing multiple functions named abs(), neither of which have uint32_t argument. As it turns out, uint32_t() - int32_t() promotes to uint32_t, where old libc abs() silently converted it back to int

Fixing casts for freq to explicitly make it a signed int when comparing with generated values, both of which are signed.

edit: noting that abs() isn't a macro and we actually want signedness

[earlephilhower]

I'm a little on the fence about converting 0x1fff->(1<<13)-1 on asthetic grounds (1fff is easier to grok for me when I think of HW register bitmasks), but generally LGTM. Thx.