net/mlx5e: Fix possible memory leak

'encap_header' is malloced and should be freed before leaving from
the error handling cases, otherwise it will cause memory leak.

Fixes: 232c0013 ("net/mlx5e: Add support to neighbour update flow")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Reviewed-by: Yuval Shaia <yuval.shaia@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>