Revisit critical section handling in MAXUSB library

usb_read_endpoint() and usb_get_setup() contain paths which call MAXUSB_ENTER_CRITICAL() but return without calling MAXUSB_EXIT_CRITICAL().

c2876eb8 fixes this, but we are not sure if the handling of the error cases is correct and if they hide other bugs. They therefore print a warning until this has been resolved.