b200: Fix for PLL setting not being applied intermittently

- Made the methods in adf4001_ctrl virtual
host/examples: rm'ed refs to d'boards, IF freq.

To reflect a reality where some USRPs don't have daughterboards,
and set_xx_bandwidth doesn't necessarily set an /IF/ bandwidth.
nirio: Address linking issues on OSX
Merge branch 'maint'

Conflicts:
	host/examples/rx_samples_to_file.cpp
Merge branch 'master' into rfnoc-devel

Conflicts:
	host/examples/rx_samples_to_file.cpp