net/mlx5: E-switch, move offloads mode callbacks to offloads file

eswitch.c is mainly for common code between legacy and offloads mode.
MAC address get and set via devlink is applicable only in offloads mode.

Hence, move it to eswitch_offloads.c file.

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