cfg80211: fix cfg80211_beacon_dup

gcc-8 warns about some obviously incorrect code:

net/mac80211/cfg.c: In function 'cfg80211_beacon_dup':
net/mac80211/cfg.c:2896:3: error: 'memcpy' source argument is the same as destination [-Werror=restrict]

From the context, I conclude that we want to copy from beacon into
new_beacon, as we do in the rest of the function.

Cc: stable@vger.kernel.org
Fixes: 73da7d5b ("mac80211: add channel switch command and beacon callbacks")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>