Adds a comment and a check to ntb_mw_get_align() so that it always fails
if the function is called before the link is up.

Also adds a comment to ntb_mw_count() to note that it may return 0 if
it is called before the link is up.

This is to prevent accidental mis-use in clients that are testing
on hardware that this doesn't matter for.
Signed-off-by: NLogan Gunthorpe <logang@deltatee.com>
Acked-by: NAllen Hubbe <Allen.Hubbe@dell.com>
Signed-off-by: NJon Mason <jdmason@kudzu.us>