[libbladeRF] API call to reset timestamps

[jynik]

Add an API call that resets the RX and TX timestamps to 0.

This allows the RX and TX modules' time tamers to be synchronized.

[jynik]

Pending further discussion - this may not be needed.

[jynik]

If both the RX and TX module are enabled, the first call to bladeRF_sync_config() will enable timestamps. The RX and TX counters should be synchronized, perhaps with a 1 cycle delta.

To reset timestamps, one can call bladerf_enable_module(dev, module, false) for both modules, and then perform the bladerf_sync_config() and bladerf_enable_module() again for them.