NTB: Enable link up and down event notification

Link-Up and Link-Down events can occur irrespective
of whether a data transfer is in progress or not.
So we need to enable the interrupt delivery for
these events early during driver load.

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