mgmt/osdp: cp: Move set PD online status to a method

When a PD is set online, we need to do other actions along with it. To
make this easier in future, move cp_set_state(pd, OSDP_CP_STATE_ONLINE)
to it's own method cp_set_online(pd).

Signed-off-by: Siddharth Chandrasekaran <sidcha.dev@gmail.com>