IB/core: Add comment for change upper netevent handling

Add comment for handling CHANGEUPPER netevent handling.
To improve code readability,
(a) move cmd definitions to its respective if-else branches,
(b) avoid single line structure definitions.

Signed-off-by: Parav Pandit <parav@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>