mgmt/osdp: Add inline methods for flag checks

The raw, flags check has become a bit excessive and has begun to affect
code readability. Provide inline functions for those accesses that are
frequent. Also, get rid of `struct osdp_cp` as it can be fully represented
by `struct osdp` itself.

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