NTB: correct ntb_spad_count comment typo

The comment for ntb_spad_count incorrectly referred to ntb_mw_count.

Signed-off-by: Aaron Sierra <asierra@xes-inc.com>
Acked-by: Allen Hubbe <Allen.Hubbe@dell.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>