yam: fix a memory leak in yam_siocdevprivate()

ym needs to be free when ym->cmd != SIOCYAMSMCS.

Fixes: 0781168e23a2 ("yam: fix a missing-check bug")
Signed-off-by: Hangyu Hua <hbh25y@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
