lmc: Use memdup_user() as a cleanup

Fix coccicheck warning which recommends to use memdup_user():
drivers/net/wan/lmc/lmc_main.c:497:27-34: WARNING opportunity for memdup_user
Generated by: scripts/coccinelle/memdup_user/memdup_user.cocci

Signed-off-by: Vasyl Gomonovych <gomonovych@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>