mac80211: Allow disabling SGI-20

This allows user-space (wpa_supplicant) to disable
short guard interval (SGI) for 20Mhz.  The SGI-40
disable option is already handled.

Signed-off-by: Ben Greear <greearb@candelatech.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>