hameg-hmo: Also start reading on timeout (workaround for USBTMC)

scpi_serial generate an POLLIN event after the requested data is returned
by the instrument. For USBTMC it is necessary to
1. send an REQUEST_DEV_DEP_MSG_IN request
2. submit an USB bulk read transfer asynchronously.

Using the synchronous libusb_bulk_read() does not generate an POLLIN event.

Solving this properly needs major surgery in spci_usbtmc_libusb.