cfg80211: Indicate backport support for sband iftype data

Add a new macro CFG80211_SBAND_IFTYPE_DATA_BACKPORT to indicate
the fact that sband iftype data has been supported for HE phy
capabilities.

Change-Id: I4cc333199308255f5795d9aa96739e05df9f209a
Signed-off-by: Wade Song <wadesong@codeaurora.org>