spi_transaction_t::cmd field

Command data, of which the length is set in the ``command_bits`` of spi_device_interface_config_t. NOTE: this field, used to be "command" in ESP-IDF 2.1 and before, is re-written to be used in a new way in ESP-IDF 3.0. Example: write 0x0123 and command_bits=12 to send command 0x12, 0x3_ (in previous version, you may have to write 0x3_12).