cfg80211: fix CFG80211_EXTRA_REGDB_KEYDIR typo

The missing CONFIG_ prefix means this macro is never defined,
leading to a possible Kbuild warning:

net/wireless/reg.c:666:20: error: 'load_keys_from_buffer' defined but not used [-Werror=unused-function]
 static void __init load_keys_from_buffer(const u8 *p, unsigned int buflen)

When we use the correct symbol, the warning also goes away.

Fixes: 90a53e44 ("cfg80211: implement regdb signature checking")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>