libbladeRF: Added "probe target" to the device probing mechanism

This is intended to allow the same device probing code to find and
present a list of bladeRF devices or FX3 bootloader devices.
libbladeRF: Removed inline specifier from libusb.c probe functions

These functions in questions are being used in a couple places. There's
no particularly good reason I see to request them to be inlined.