Fixed type in libusb0 backend.
Fixed type in libusb0 backend.

AlanJAS/pyusb,karlp/pyusb,h3nj3/pyusb,kopchik/pyusb,swinman/pyusb,rabryan/pyusb,h3nj3/pyusb,AlanJAS/pyusb,kopchik/pyusb,makerbot/pyusb,makerbot/pyusb,rabryan/pyusb,swinman/pyusb,karlp/pyusb