kaweth: Fix memory leak in kaweth_control()

kaweth_control() never frees the buffer that it allocates for the USB
control message.  Test case:

while :; do ifconfig eth2 down ; ifconfig eth2 up ; done

This is a tiny buffer so it is a slow leak.  If you want to speed up the
process, you can change the allocation size to e.g. 16384 bytes, and it
will consume several megabytes within a few minutes.

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
Acked-by: Oliver Neukum <oliver@neukum.org>
Signed-off-by: David S. Miller <davem@davemloft.net>