Fix new incorrect error return from do_md_stop.

Recent commit c8c00a6915a2e3d10416e8bdd3138429beb96210
changed the exit paths in do_md_stop and was not quite
careful enough.  There is one path were 'err' now needs
to be cleared but it isn't.
So setting an array to readonly (with mdadm --readonly) will
work, but will incorrectly report and error: ENXIO.

Signed-off-by: NeilBrown <neilb@suse.de>