USB: mon: Allow to use usbmon without debugfs

Do not bail out with an error in mon_text_init() if debugfs is not
available, instead just return 0 and let mon_init() go ahead with
loading the binary API. Return -ENOMEM in case debugfs_create_dir()
fails for other reasons. Later, it is enough to check for mon_dir
not set.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: Pete Zaitcev <zaitcev@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>