mgmt/mcumgr/lib: Fix os_mgmt_echo returning Cbor error

The os_mgmt_echo is supposed to return MGMT type error not
Cbor; there was missing translation.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>

subsys/mgmt/mcumgr/lib/cmd/os_mgmt/src/os_mgmt.c

diff --git a/subsys/mgmt/mcumgr/lib/cmd/os_mgmt/src/os_mgmt.c b/subsys/mgmt/mcumgr/lib/cmd/os_mgmt/src/os_mgmt.c
index daca3b2..0f060b5 100644
--- a/subsys/mgmt/mcumgr/lib/cmd/os_mgmt/src/os_mgmt.c
+++ b/subsys/mgmt/mcumgr/lib/cmd/os_mgmt/src/os_mgmt.c
@@ -46,7 +46,7 @@
 
 	err = cbor_read_object(&ctxt->it, attrs);
 	if (err != 0) {
-		return MGMT_ERR_EINVAL;
+		return mgmt_err_from_cbor(err);
 	}
 
 	echo_buf[sizeof(echo_buf) - 1] = '\0';