fsl/fman: fix a leak in tgec_free()

We set "tgec->cfg" to NULL before passing it to kfree().  There is no
need to set it to NULL at all.  Let's just delete it.

Fixes: 57ba4c9b ("fsl/fman: Add FMan MAC support")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>