NTB: Make _addr functions optional in the API

The functions ntb_peer_db_addr and ntb_peer_spad_addr were required by
the api.  The functions already support returning an error, so any
existing calling code should already check for it.  Any existing code
using drivers that implement the functions will not be affected.

Signed-off-by: Allen Hubbe <Allen.Hubbe@emc.com>
Signed-off-by: Jon Mason <jdmason@kudzu.us>