treewide: mgmt: mcumgr: Change "ret" to "err" for SMP version 2

This is a stable API treewide change changing the newly introduced
"ret" response to "err" as it was overlooked that the shell_mgmt
group already used "ret" to return the exit code of the command
and this created a collision. Since SMP version 2 was only recently
introduced, there should not be any public implementations of it
as of yet, but the original function has been kept and marked as
deprecated.

Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>