iwlwifi: check short GI support per-station rather than globally

Short guard interval support is a local per-station parameter not a global
per-NIC parameter.  (mac80211 will correctly remove SGI support from station
capabilities if the BSS does not permit it).  This patch removes the short GI
support bitfield from the global iwl_ht_info struct and properly uses
per-station HT capabilities during rate selection.

Signed-off-by: Daniel C Halperin <daniel.c.halperin@intel.com>
Acked-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>