libbladeRF: Ensure dev->cal.dc_<rx|tx> pointers are NULL after free

While reviewing config.c, it was noted that before attempting to
load a DC cal table, the old one (if present) is first freed.

While the current code doesn't provide the ability to unload and reload
DC calibration tables, this change is intended to avoid double free
issues when such functionality is added.
libbladeRF: Load LMS DC cal registers from table(s) and related cleanup

Although I/Q DC offsets were being applied when tuning the device's
center frequency, the LMS DC registers' calibration results were not being
applied from the table, making the I/Q DC offsets from the table less
effective.

These values are now written to the LMS during the init_device() call,
which occurs once after the FPGA is loaded, if DC calibration tables are
loaded.  If only an RX or only a TX table is available, all of the
register values included with the table are used. Otherwise, only
relevant items from each table are used and the LPF Tuning value is
checked for a discrepancy.

To achieve this, the config_load_all() function has been split into
the DC cal table loading and FPGA loading functions, because the DC cal
table must be loaded prior to init_device(), whereas the FPGA loading is
intended to be the last call made.

Cleanup items here were just removal of unused bitrot:
    - Removal of the load_calibration_table() function. This was
      a vestige of earlier development efforts. The functionality here
      is performed in dc_cal_table.c and in the updates to
      init_device().

    - Removal of the lms_dc_ref_vals structure. This was replaced by
      bladerf_lms_dc_cals in a1b9f80.