net/mlx5: E-Switch, Refactor offloads flow steering init/cleanup

E-switch offloads mode initialize/cleanup multiple steering related
entities (flow table/group). Refactor these operations to internal
helper functions for better block design.

This patch doesn't change any functionality.

Signed-off-by: Bodong Wang <bodong@mellanox.com>
Reviewed-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>