LibbladeRF support for setting MIMO mode

[polygon]

There is no libbladeRF command yet to set the MIMO mode. It is implemented in bladeRF-cli mimo.c as a series of bladerf_si5338_write calls. To avoid replicating this code in third party programs, there should be a bladerf_mimo_mode call or similar.

Furthermore, since the calls in mimo.c change different registers when switching to master and slave mode, switching a bladeRF from master to slave or the other way round is probably not having the desired effect. That case is likely not very relevant, though.

[jynik]

This has been added as of libbladeRF v1.7.0, under this section of the API.

bladeRF-cli v1.4.0 uses this in the set smb_mode and print smb_mode implementations.