net/mlx5: E-Switch prepare functions change handler to be modular

Eswitch function change handler will service multiple type of events for
VFs and non VF functions update.
Hence, introduce and use the helper function
esw_vfs_changed_event_handler() for handling change in num VFs to improve
the code readability.

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