iwlwifi: add FIFO usage for 5000

This is part of the code, but the comment doesn't have it,
add pointers to the code and the FIFO usage for 5000 and
up.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>