spi_transaction_t::addr field

Address data, of which the length is set in the ``address_bits`` of spi_device_interface_config_t. NOTE: this field, used to be "address" in ESP-IDF 2.1 and before, is re-written to be used in a new way in ESP-IDF3.0. Example: write 0x123400 and address_bits=24 to send address of 0x12, 0x34, 0x00 (in previous version, you may have to write 0x12340000).