iwlwifi: fix a typo

We never have four chains, but let's fix the typo
while we noticed it. You count 0, 1, 2, 3, not
0, 1, 2, 4 :)

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>