NTB: handle link down event correctly

Link-Up and Link-Down are mutually exclusive events.
So when we receive a Link-Down event, we should also
clear the bitmask for Link-Up event in peer_sta.

Signed-off-by: Arindam Nath <arindam.nath@amd.com>
Signed-off-by: Jon Mason <jdmason@kudzu.us>