net/mlx5e: Fix error handling when refreshing TIRs

Previously, a false positive would be caught if the TIRs list is
empty, since the err value was initialized to -ENOMEM, and was only
updated if a TIR is refreshed. This is resolved by initializing the
err value to zero.

Fixes: b676f653

 ("net/mlx5e: Refactor refresh TIRs")
Signed-off-by: Gavi Teitz <gavi@mellanox.com>
Reviewed-by: Roi Dayan <roid@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>