SPISettings no longer constexpr

[jgromes]

Describe the bug
As discussed in #2191, the current SPISettings handling is incorrect, it should be made constexpr where applicable.

To Reproduce
Attempt to run something using SubGhz.spi_settings during object initialization, e.g. some RadioLib STM32WL Example.

Additional context
The recent changes to SPI library in 392469a introduced couple of issues. One was addressed by #2191, however, there is also a continued issue with constexpr being removed from SPISettings. To fix:

1. d61ad83 (merged in fix(SubGhz): SPISettings not properly defined #2193) should be reverted
2. SPISettings constructors should be made constexpr again, which will require removing init_AlwaysInline + init_MightInline and doing initialization in a simple constructor.