net/mlx5e: Set port trust mode to PCP as default

The hairpin offload code has dependency on the trust mode being PCP.

Hence we should set PCP as the default for handling cases where we are
disallowed to read the trust mode from the FW, or failed to initialize it.

Fixes: 106be53b ('net/mlx5e: Set per priority hairpin pairs')
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Reviewed-by: Parav Pandit <parav@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>