xconfig: merge code path to conf_write()

Avoid to have multiple path saving the config. This fixes an error check
miss when the window is being closed and the user requested the config
to be written.

Reported-by: Hiromu Yakura <hiromu1996@gmail.com>
Pointed-out-by: Michal Marek <mmarek@suse.cz>
Signed-off-by: Michal Marek <mmarek@suse.cz>