Change v4l2_scan_controls_enumerate_menu to not perror on null ioctl.

See http://www.raspberrypi.org/forum/viewtopic.php?f=43&t=65026 for detail. Basically, this change copies the same procedure that v4l2-ctl uses in v4l2-ctl-common.cpp/print_qctrl  (http://git.linuxtv.org/v4l-utils.git/tree/refs/heads/master:/utils/v4l2-ctl).