[PATCH] iwlwifi: fix add_station to avoid FW error

There were a few Firmware errors reported the most reproducible
http://bughost.org/bugzilla/show_bug.cgi?id=1471



The root cause is rate_n_flags isn't set anymore. This patch fixes
the problem.

Signed-off-by: Ian Schram <ischram@telenet.be>
Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>