nl80211: fix MAC address validation

MAC addresses have a fixed length. The current
policy allows passing < ETH_ALEN bytes, which
might result in reading beyond the buffer.

Cc: stable@vger.kernel.org
Signed-off-by: Eliad Peller <eliad@wizery.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>