iwlwifi: don't switch to SGI if not supported by AP

This patch fixes SGI support. RS didn't look at the capabilities of the AP
before switching to SGI, this should lead to a stall in the traffic with an
AP that doesn't support SGI.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Guy Cohen <guy.cohen@intel.com>
Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>