net/mlx5e: Fix possible non-initialized struct usage

If mlx5e_devlink_port_register() fails, driver may try to register
devlink health TX and RX reporters on non-registered devlink port.

Instead, create health reporters only if mlx5e_devlink_port_register()
does not fail. And destroy reporters only if devlink_port is registered.

Also, change mlx5e_get_devlink_port() behavior and return NULL in case
port is not registered to replicate devlink's wrapper when ndo is not
implemented.

Signed-off-by: Vladyslav Tarasiuk <vladyslavt@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>