hog-lib: Fix not handling BT_ATT_OP_HANDLE_NFY_MULT

This is a temporary fix for not handling BT_ATT_OP_HANDLE_NFY_MULT in
GAttr so the code will use g_attrib_attach_client to attach the
bt_gatt_client instance which is then used to register notifications
including those sent with BT_ATT_OP_HANDLE_NFY_MULT.

Fixes: https://github.com/bluez/bluez/issues/71