Short circuit flushing USB endpoints if no data to send.
Short circuit flushing USB endpoints if no data to send.

This is a fairly significant speedup - the cost of switching the active
endpoint was high and we were doing it more often than required.

mgiannikouris/vi-firmware,ene-ilies/vi-firmware,openxc/vi-firmware,openxc/vi-firmware,mgiannikouris/vi-firmware,ene-ilies/vi-firmware,openxc/vi-firmware,mgiannikouris/vi-firmware,openxc/vi-firmware,ene-ilies/vi-firmware,ene-ilies/vi-firmware,mgiannikouris/vi-firmware