mcumgr: fix invalid condition for calling img_mgmt_dfu_stopped

In other places, img_mgmt_dfu_stopped() is called when
a failure occurs and the DFU cannot be continued. In this
place, however, the function is called on success which
does not seem to be correct.

Signed-off-by: Damian Krolik <damian.krolik@nordicsemi.no>