net/mlx5: Fix missing return value in mlx5_devlink_eswitch_inline_mode_set()

The return value is missing in this code scenario, add the return value
'0' to the return value 'err'.

Eliminate the follow smatch warning:

drivers/net/ethernet/mellanox/mlx5/core/eswitch_offloads.c:3083
mlx5_devlink_eswitch_inline_mode_set() warn: missing error code 'err'.

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Fixes: 8e0aa4bc ("net/mlx5: E-switch, Protect eswitch mode changes")
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>