mgmt/osdp: Cleanup log messages and return codes

The log lines in CP and PD had a prefix such as "CP: " and "PD: " that
does not add too much value as a given device an either be CP or PD
only. This patch removes those and enhances some other log lines while
at it.

It also adds a enum for return values throughout the module to improve
code quality.

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