applications: nrf_desktop: Fix returning error in MCUmgr callbacks

Change fixes returning error in MCUmgr callbacks. This prevents
assertion failures when the error is returned by the application module.

Jira: NCSDK-22964

Signed-off-by: Marek Pieta <Marek.Pieta@nordicsemi.no>

applications/nrf_desktop/src/modules/dfu_mcumgr.c

@@ -69,7 +69,8 @@ static enum mgmt_cb_return mcumgr_img_mgmt_cb(uint32_t event,
 	k_work_reschedule(&dfu_timeout, DFU_TIMEOUT);
 	if (IS_ENABLED(CONFIG_DESKTOP_DFU_LOCK) && dfu_lock_claim(&mcumgr_owner)) {
 		(void)k_work_cancel_delayable(&dfu_timeout);
-		return MGMT_ERR_EACCESSDENIED;
+		*rc = MGMT_ERR_EACCESSDENIED;
+		return MGMT_CB_ERROR_RC;
 	}
 
 	if (IS_ENABLED(CONFIG_MCUMGR_TRANSPORT_BT) &&
@@ -96,7 +97,8 @@ static enum mgmt_cb_return mcumgr_os_mgmt_reset_cb(uint32_t event,
 	k_work_reschedule(&dfu_timeout, DFU_TIMEOUT);
 	if (IS_ENABLED(CONFIG_DESKTOP_DFU_LOCK) && dfu_lock_claim(&mcumgr_owner)) {
 		(void)k_work_cancel_delayable(&dfu_timeout);
-		return MGMT_ERR_EACCESSDENIED;
+		*rc = MGMT_ERR_EACCESSDENIED;
+		return MGMT_CB_ERROR_RC;
 	}
 
 	return MGMT_CB_OK;