mgmt/mcumgr: Fix image erase returning MGMT_ERR_EMSGSIZE

In case when CONFIG_MCUMGR_SMP_LEGACY_RC_BEHAVIOUR=n then
the image erase command would incorrectly return MGMT_ERR_EMSGSIZE,
even though no such error occurred.

Fixes #57319

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>
(cherry picked from commit 00f1639e)