libbladeRF: Removed unnecessary lna read/writes in lms_calibrate_dc()

The LNA is left the same throughout this function and there are no
desired side-effects from writing it. Removing it to simplify upcoming
fixes and cleanup.
libbladeRF: Started cleanup of lms_calibrate_dc() function

This first stab at cleaning up the lms_calibration_dc() function
focuses on separating out the backup/restore and init/deinit portions
to separate (inline) functions for better readability.

The objective here is mainly to separate logical components from this
function to simplify some upcoming changes.
libbladeRF: Updated LMS RMW operations in dc cal to use lms_set/clr()

While refactoring the DC calibration code in order to more easily
integrate some upcoming "retry" changes (and improve readability),
it was noted that there were a number of simply RMW operations that
could be replaced with lms_clr() and lms_set().

The DC calibration-related functions have been updates to use these
instead of a manual LMS_READ(), modify, LMS_WRITE() operations.

Removed the associated "TODO" for this task, as it does not appear
any remaining sections would benefit from this change. There are some
slightly more complex "modify" portions of RMW operations - they're
probably most readable as is.
libbladeRF: Added LMS DC cal retries with gain adjustments

Instead of bailing out and reporting "Bad DC_REGVAL" when an LMS DC
calibration routine fails to converge, we now attempt to reduce gains
and retry the calibration.

Generally, the convergence failures appear to be caused by signals
being too noisy for the internal calibration algorithm(s).

On a relatively small sample set, this has shown to alleviate
convergence failures, perhaps with a trade-off with some accuracy in the
DC cal.
libbladeRF: Renamed lms_clr() to lms_clear()

Per code review feedback - abbreviated names are not preferred.