IB/hfi1: Send 'reboot' as planned down remote reason

On host shutdown, driver sends 'SMA_Disabled' as a reason
for link down. This is incorrect.

Send 'reboot' as a linkdown reason.

Signed-off-by: Jan Sokolowski <jan.sokolowski@intel.com>
Reviewed-by: Jakub Byczkowski <jakub.byczkowski@intel.com>
Signed-off-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>