net/mlx5: Bridge, obtain core device from eswitch instead of priv

Following patches in series will pass bond device to bridge, which means
the code can't assume the device is mlx5 representor. Moreover, the core
device can be easily obtained from eswitch instance, so there is no reason
for more complex code that obtains struct mlx5_priv from net_device in
order to use its mdev. Refactor the code to use esw->dev instead of
priv->mdev.

Signed-off-by: Vlad Buslov <vladbu@nvidia.com>
Reviewed-by: Roi Dayan <roid@nvidia.com>
Reviewed-by: Mark Bloch <mbloch@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>