net/mlx5: E-Switch, Move legacy code to a individual file

Currently eswitch offers two modes. Legacy and offloads.
Offloads code is already in its own file eswitch_offloads.c

However eswitch.c contains the eswitch legacy code and common
infrastructure  code.

To enable future extensions and to better manage generic common eswitch
infrastructure code, move the legacy code to its own legacy.c file.

Signed-off-by: Parav Pandit <parav@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>