Fixed a bug when using SPI / ESP32
The default SPI object ( _spi{&SPI} ) was missing for one of the SPI constructors. Despite this, it strangely worked on an Arduino Nano and therefore I had not noticed.
MCP23017(uint8_t cs, uint8_t rp, uint8_t addr) : _spi{&SPI}, SPI_Address{addr}, resetPin{rp}, csPin{cs}, useSPI{true} {}