bladeRF-flash: cast callback fn to proper type

Fixes werrors:

/bladerf/bladeRF/host/utilities/bladeRF-flash/src/main.c:240:13:
    error: passing argument 7 of 'libusb_hotplug_register_callback'
    from incompatible pointer type [-Werror]
/usr/i686-w64-mingw32/include/libusb-1.0/libusb.h:1928:17:
    note: expected 'libusb_hotplug_callback_fn' but argument is of type
    'int (*)(struct libusb_context *, struct libusb_device *, enum libusb_hotplug_event,  void *)'
Merge pull request #263 from rtucker/win-mingw-fixes-bladerf-flash

bladeRF-flash: cast callback fn to proper type
WIP: Modify the RXVGA2 calibration routines.
Calibration update.
libbladeRF: Moved LPF DC tuning value write into address loop

This is just for readability. There's no functional change, since
there's only 1 address to loop over anyway.
libbladeRF: Added register settings from LMS FAQ to DC calibration fns

The following changes have been added to lms_dc_cal_loop() and
lms_calibrate_dc(), based upon recommendations from the LMS 6002D FAQ
(version 1.0r10).

    - Use the DC_REGVAL to determine success or failure, as DC_LOCK
      is reported (in the FAQ) to be unreliable. Retry if the result is
      31, setting the count to DC_CNTVAL to 0.

    - Enable DC comparators during DC calibration (default), and
      disable them upon completion to address linearity issues.

    - Disable DACs while performing TXLPF DC calibration
bladeRF-cli: [WIP] First cut at RX DC auto calibration

Includes a workaround for convergence issues - further investigation
required. General cleanup and documentation changes still required.
bladeRF-cli: [WIP] Use fewer buffers for dc cal, start with cal at 0,0
bladeRF-cli: [WIP] Added TX DC cal operation. Further cleanup required.
bladeRF-cli: Check for divide-by-zero in interpolation function
libbladeRF: Be explicit with RXVGA2 cal.

It was seen that the VCM will sometimes come back in a bad state.
This register has a very well known value that it should be.  Setting
it to be these explicit values seems to help alleviate putting the
LMS is a bad state.

Investigation is required to figure out why the VCM is coming back
scrogged.
bladeRF-cli: Extend out DC, GAIN and PHASE max/min.
bladeRF-cli: Fixed uninitialized error value for TX DC cal
bladeRF-cli: Added RX DC cal option that skips LMS calibration
libbladeRF: [WIP] Disable LNA while performing LMS DC tuning
libbladeRF: [WIP] Perform LMS RX DC cal during device initialization
bladeRF-cli: cal dc_rx/tx no longer perform LMS calibration
bladeRF-cli: Attempt to narrow RX DC auto cal value, removed dummy RX
bladeRF-cli: Fixed 'cal dc_tx', broken in c0941d9
bladeRF-cli: Removed unused function to alleviate build warning