HID: register debugfs entries before adding device

Register debugfs entries before calling device_add() so debugfs entries are
already present when HID driver's probe function gets called on device hotplug.

Also undo debugfs entry registration if device_add() fails so status
HID_STAT_ADDED and debugfs registration status remain consistent and we don't
leak the debugfs entries.

Signed-off-by: Bruno Prémont <bonbons@linux-vserver.org>
Cc: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>