iwlagn: improve rate scale table search

iwlagn rate scaling will periodically search other rate scale
tables to switch to the best table regarding performance. In the past
the number of search tables were 3. Every time the rate scale algorithm
goes through these available tables in will stay in current table for
some time before start searching again. Recent driver support more
feature and antenna, so we have more tables to search. This patch make
sure we go through all available tables.

Signed-off-by: Mohamed Abbas <mohamed.abbas@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>