DFS: Do not use cf1 to override freq for 20 MHz channels

DFS: Do not use cf1 to override freq for 20 MHz channels

NL80211_ATTR_CENTER_FREQ1 is defined to be used for anything but 20 MHz
bandwidth, so it could be unset for 20 MHz channels. Do not use it to
override center frequency from NL80211_ATTR_WIPHY_FREQ (if available)
for 20 MHz channels to avoid clearing frequency.

Signed-hostap: Jouni Malinen <j@w1.fi>