DFS offload: Use hostapd_is_dfs_required() to check if DFS required

DFS offload: Use hostapd_is_dfs_required() to check if DFS required

hostapd_handle_dfs_offload() is the DFS handler for the offloaded case,
in which ieee80211_is_dfs() is used to check if the configured frequency
requires DFS or not.

When the configured channel width is not 20 (e.g., 160),
ieee80211_is_dfs() will not checked adjacent freqs, so it possibly makes
wrong conclusion for whether DFS is required.

hostapd_is_dfs_required() does similar thing with ieee80211_is_dfs()
except it supports checking whether the configured frequency and its
adjacent frequencies require DFS. So hostapd_is_dfs_required() is a more
robust and better option than ieee80211_is_dfs() to check DFS.

The issue is hostapd_is_dfs_required() is for non-offload case due to
the check of the configuration parameter ieee80211h. Add a check for
WPA_DRIVER_FLAGS_DFS_OFFLOAD to make it support the DFS offload case
(i.e., ieee80211h=0) as well.

For example, configuring the AP to start at freq=5240 with channel width
160:
- Existing hostapd checks freq=5240 is non-DFS, hence skip DFS CAC and
  transition to AP-Enabled which volatiles DFS-RADAR detection.

  LOG: "hostapd : hostapd_handle_dfs_offload: freq 5240 MHz does not
        require DFS. Continue channel/AP setup"

- This commit checks freq=5240 and its adjacent freqs are DFS required,
  hence remains in DFS state until DFS CAC completed.

  LOG: "hostapd : hostapd_handle_dfs_offload: freq 5240 MHz requires
        DFS for 4 chans"

Signed-off-by: Hu Wang <huw@codeaurora.org>