amt: Fix NULL but dereferenced coccicheck error

Eliminate the following coccicheck warning:
./drivers/net/amt.c:2795:6-9: ERROR: amt is NULL but dereferenced.

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Yang Li <yang.lee@linux.alibaba.com>
Signed-off-by: David S. Miller <davem@davemloft.net>