ipc: fix return code of icbmsg backend send operation.

This commit fixes the issue where a serialization
error was reported after properly sending a data with 'icbmsg' backend.

The icbmsg send function's return code is set to
the sent data's len as in other backends.
The related docs were fixed and updated.

Signed-off-by: Piotr Koziar <piotr.koziar@nordicsemi.no>