mgmt/osdp: Make OSDP slab init safer

slab_buff size is calculated based on size of osdp_ephemeral_data union
so it is safer to use same structure for slab init.
Changes in osdp_event or osd_cmd structures can cause crashes without this.

Signed-off-by: Arkadiusz Wadowski <wadowski.arkadiusz@gmail.com>