yam: fix a memory leak in yam_siocdevprivate()

[ Upstream commit 29eb3154 ]

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

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