mgcp-client: Refactor system keeping old users not calling mgcp_client_pool_config_write() working

Move the regular writing logic to a subfunction, and raise the flag only
on one of them.
This simplifies the code paths but not marking stuff true and false over
the same code path.

Change-Id: I070798863f2bdc253df004a63262d4bcd529348f
1 file changed