libbladeRF: Fixed # samples parameter on initial  TX callbacks (libusb backend)
fx3_firmware: speed up FPGA image bitflipping by using a lookup table

The bitflipping look takes about 180 instruction cycles to flip 1 byte. The new
method takes less than 5 clock cycles, making both images load times seem
almost instantenous. Further optimizations can be made by adding a Duff's
machine.