host: rfnoc_replay_samples_from_file.cpp add multi-channel support

[urima]

rfnoc_replay_samples_from_file.cpp is a great example.
It enable to upload a file to the USRP dram and replay it at the maximum sample rates without ant U/D/O issues.
It will be more useful if it support multichannel, so one can upload two or more files (depend or number of TX ports of the USRP) to the dram and replay it all at same time.

[mbr0wn]

@urima we also try and keep our examples as simple as possible, so they can serve as good examples for people writing their own code. We are currently reviewing our example portfolio, so maybe this change is useful, but we might also make it an example for people to write their own applications.