mgmt/osdp: pd: Rework state machine to handle multiple packets

After the check_packet() split from decode_packet() for handling multiple
packets in the RX buffer, PD state machine needed to me modified to
start handling it correctly. While at it, simplify the states as PD does
not have as much state anyways.

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