Workaround for possible bug in the SAF1761 chip. Wait 125us before
re-using a hardware propritary transfer descriptor, PTD, in USB host
mode. If the PTD's are recycled too quickly, it has been observed that
the hardware simply fails to schedule the requested job or resets
completely disconnecting all devices.
Optimise the bit searching loops, by quickly skipping the 16 first set
bits if all the 16 first bits are set. This way the worst case
searching time is reduced from 32 to 16 cycles.