net/mlx5e: Do not use err uninitialized in mlx5e_rep_add_meta_tunnel_rule()

Clang warns:

  drivers/net/ethernet/mellanox/mlx5/core/en_rep.c:481:6: error: variable 'err' is used uninitialized whenever 'if' condition is false [-Werror,-Wsometimes-uninitialized]
          if (IS_ERR(flow_rule)) {
              ^~~~~~~~~~~~~~~~~
  drivers/net/ethernet/mellanox/mlx5/core/en_rep.c:489:9: note: uninitialized use occurs here
          return err;
                ^~~
  drivers/net/ethernet/mellanox/mlx5/core/en_rep.c:481:2: note: remove the 'if' if its condition is always true
          if (IS_ERR(flow_rule)) {
          ^~~~~~~~~~~~~~~~~~~~~~~
  drivers/net/ethernet/mellanox/mlx5/core/en_rep.c:474:9: note: initialize the variable 'err' to silence this warning
          int err;
                ^
                  = 0
  1 error generated.

There is little reason to have the 'goto + error variable' construct in
this function. Get rid of it and just return the PTR_ERR value in the if
statement and 0 at the end.

Fixes: 430e2d5e

 ("net/mlx5: E-Switch, Move send to vport meta rule creation")
Link: https://github.com/ClangBuiltLinux/linux/issues/1695
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Link: https://lore.kernel.org/r/20220825180607.2707947-1-nathan@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>