cfg80211: Add backport flag for enabling DFS offload

Upstream commit 13cf6dec ("cfg80211/nl80211: add DFS offload flag")
adds wiphy EXT_FEATURE flag to indicate that HW or driver does all DFS
actions by itself. Add backport flag for the same.

Change-Id: Ic79c10c82c6ba4fb5960c9f5a71fc0e69eeeb270
CRs-fixed: 2265562
Signed-off-by: Peng Xu <pxu@codeaurora.org>