mgmt/osdp: phy: Move packet checks out of decode_packet

To handle multiple packets in RX buffer, we need to perform checks on
the buffer first and determine the length of one packet and extact only
that many bytes from the RX buffer instead of pulling everything at
once and failing if there are bytes from the next packet in the buffer.

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