net/mlx5: E-Switch, Fix uninitialized variable modact

The variable modact is not initialized before used in command
modify header allocation which can cause command to fail.

Fix by initializing modact with zeros.

Addresses-Coverity: ("Uninitialized scalar variable")
Fixes: 8f1e0b97 ("net/mlx5: E-Switch, Mark miss packets with new chain id mapping")
Signed-off-by: Maor Dickman <maord@nvidia.com>
Reviewed-by: Roi Dayan <roid@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>