refactor(TMC2130_Basics): optimize SPI flush loops

[Chris0xdeadbeef]

This PR combines two commits refactoring the SPI flush loops in TMC2130_Basics:

• Replaced magic numbers with constexpr uint8_t BYTES_TO_FLUSH = 4
• Changed loop counters from int to uint8_t
• Replaced post-increment i++ with pre-increment ++i for consistency

No functional changes, just code clean-up and minor performance improvements.