net/mlx5: Fix a compilation warning in events.c

Eliminate the following compilation warning:

drivers/net/ethernet/mellanox/mlx5/core/events.c: warning: 'error_str'
may be used uninitialized in this function [-Wuninitialized]:  => 238:3

Fixes: c2fb3db2

 ("net/mlx5: Rework handling of port module events")
Signed-off-by: Tariq Toukan <tariqt@mellanox.com>
Reviewed-by: Mikhael Goikhman <migo@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>