net/mlx5e: Fix error return code in mlx5e_tc_esw_init()

Fix to return negative error code from the mlx5e_tc_tun_init() error
handling case instead of 0, as done elsewhere in this function.

This commit also using 0 instead of 'ret' when success since it is
always equal to 0.

Fixes: 8914add2 ("net/mlx5e: Handle FIB events to update tunnel endpoint device")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>