dfu: Allow the DFU plugin to write Intel HEX firmware
dfu: Allow the DFU plugin to write Intel HEX firmware

This was being used in the A3BU device test, which broke when we
switched to fu_firmware_new_from_gtypes().

fwupd/fwupd,fwupd/fwupd,fwupd/fwupd,fwupd/fwupd