HID: add hid_hw_open/close/power() handlers

Instead of exposing the guts of hid->ll_driver relationship to HID
sub-drivers provide these helpers to encapsulate the details.

Signed-off-by: Dmitry Torokhov <dtor@mail.ru>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>