mac80211: don't kmalloc 16 bytes

Since this small buffer isn't used for DMA,
we can simply allocate it on the stack, it
just needs to be 16 bytes of which only 8
will be used for WEP40 keys.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>