NTB: Make link-state API being declared first

Since link operations are usually performed before memory window access
operations, it's logically better to declare link-related API before any
of MW/Doorbell/Scratchpad methods.

Signed-off-by: Serge Semin <fancer.lancer@gmail.com>
Signed-off-by: Jon Mason <jdmason@kudzu.us>