[spi]: Keep constexpr constructor

[fpistm]

to ensures that constructing an SPISettings object can be done at compiletime.

This PR also:

• revert the change made in SubGhz library.
• clean up SubGhzSPI class since internal CS pin removed.
• Remove skip receive from SPISettings as this is not a settings.
• Allow to skip receive during transfer (SPI: improvements for SPI library #912).

Fixes #2201.

[fpistm]

Tested with Radiolib with success using Nucleo WL55JC1 and LoRa E5 mini.

Note to have generic sketch example use the LORAWAN_RFSWITCH_* definition defined in the variant_*.h:

static const uint32_t rfswitch_pins[] = { LORAWAN_RFSWITCH_PINS
#if LORAWAN_RFSWITCH_PIN_COUNT == 2
                                          ,
                                          RADIOLIB_NC
#endif
};

static const Module::RfSwitchMode_t rfswitch_table[] = {
  { STM32WLx::MODE_IDLE, { LORAWAN_RFSWITCH_OFF_VALUES } },
  { STM32WLx::MODE_RX, { LORAWAN_RFSWITCH_RX_VALUES } },
  { STM32WLx::MODE_TX_LP, { LORAWAN_RFSWITCH_RFO_LP_VALUES } },
  { STM32WLx::MODE_TX_HP, { LORAWAN_RFSWITCH_RFO_HP_VALUES } },
  END_OF_MODE_TABLE,
};