mgmt/osdp: More error handling and other cleanups

cp_build_command returns the length of the built packet and early
returns for errors. This means there is no need to keep track of return
code in a separate variable -- if we reached the end, we succeeded.

Along with this change, some more changes to enhance maintainability and
readability are also done.

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