David Zaslav has a simple message for Paramount Skydance chief David Ellison: Get serious or go home.The hard-charging Warner Bros. Discovery CEO has relayed to Ellison, who reportedly has been floating a lowball bid of around $20 a share and suggesting he might go hostile – that he needs to put up more money, possibly upwards of $30 a share to close a deal, The Post has learned.Ellison, just months after he purchased Paramount in a deal valued at $8 billion, is expected to finally make an official offer fo ...