Host: Move LMS6002D control class to  separate source file.
FPGA: The fix for multiplexer for the RX and TX frontends was added.
Host: Change LMS interace from separate Rx and Tx classes to a single XCVR class.
Host: Rename lms6002d_dev::lms_txrx_pll_tune() to lms6002d_dev::txrx_pll_tune() for better consistency.
Host: Move LMS interface in LMS2006D daughter board class into a variable for cleaner code.
ZPU: Fix a typo in a comment.
Host: Adding LMS auto-calibration on UHD initialization.

We should add a way to call certain parts of auto-calibration manually. E.g. this would be helpful after Tx VGA gain changes. Right now we calibrate for Tx VGA1 gain -10 which may give imperfect results for other gain settings.
Host: Don't set Tx gains in lms6002d_dev::init().