Bluetooth: L2CAP: Add missing `req_mtu` assert in `l2cap_ecred_conn_req`

There is an assert that `req_psm` is the same for all channels in the
same connection request because HCI requires this. The same is true for
`req_mtu`. This adds the obviously missing assert.

Signed-off-by: Aleksander Wasaznik <aleksander.wasaznik@nordicsemi.no>