ep_rx_pcs: fix an unsafe transfer of an_rx_val

The autonegotiation state machine uses an_rx_val in the sys clock domain.
The rx pcs passes the an_rx_val to the sys domain using a handshake.
Unfortunately, the value can change even after the handshake goes high.
On Altera, this led to the autonegotiation state machine entering state "0".
... from which it would never leave and the link stayed dead forever.