net/mlx5e: Use short attribute form when adding/deleting offloaded TC flows

Instead of going through flow->nic/esw_attr for each usage, assign
an attr pointer per the context (nic or esw) and use that.

This patch doesn't add any functionality.

Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Reviewed-by: Roi Dayan <roid@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>