net/mlx5: E-Switch, Centralize repersentor reg/unreg to eswitch driver

Eswitch has two users: IB and ETH. They both register repersentors
when mlx5 interface is added, and unregister the repersentors when
mlx5 interface is removed. Ideally, each driver should only deal with
the entities which are unique to itself. However, current IB and ETH
drivers have to perform the following eswitch operations:

1. When registering, specify how many vports to register. This number
   is the same for both drivers which is the total available vport
   numbers.
2. When unregistering, specify the number of registered vports to do
   unregister. Also, unload the repersentors which are already loaded.

It's unnecessary for eswitch driver to hands out the control of above
operations to individual driver users, as they're not unique to each
driver. Instead, such operations should be centralized to eswitch
driver. This consolidates eswitch control flow, and simplified IB and
ETH driver.

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>