net/mlx5: add OS specific flow type selection

net/mlx5: add OS specific flow type selection

In current implementation the flow type (DV/Verbs) is selected
using dedicated function flow_get_drv_type().

This patch adds OS specific function mlx5_flow_os_get_type(), to
allow OS specific flow type selection.
The new function is called by flow_get_drv_type(), and if it returns a
valid value (DV/Verbs) no more logic is required.
Otherwise the existing logic is executed.

Signed-off-by: Dekel Peled <dekelp@mellanox.com>
Acked-by: Matan Azrad <matan@mellanox.com>