cfg80211: fix warning when using WEXT for IBSS

Fix kernel warning when using WEXT for configuring ad-hoc mode,
e.g.  "iwconfig wlan0 essid test channel 1"

WARNING: at net/wireless/chan.c:373 cfg80211_chandef_usable+0x50/0x21c [cfg80211]()

The warning is caused by an uninitialized variable center_freq1.

Cc: stable@vger.kernel.org
Signed-off-by: Bruno Randolf <br1@einfach.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>