UFC's Jeremy Stephens -- Fires Back At Conor McGregor ... 'I'll Mop the Floor with You'

UFC fighter Jeremy Stephens vows REVENGE against Conor McGregor for trashing him at the UFC 205 media event -- telling TMZ Sports he'll "mop the floor" with the Irish superstar.  Conor dissed the hell out of Stephens during the event -- when he…