octeontx2-af: Unregister cgx event callbacks gracefully

Added provision to unregister cgx event callbacks.
This enables the exit path to ensure event callbacks are
unregistered before workqueues get destroyed.

Signed-off-by: Linu Cherian <lcherian@marvell.com>
Signed-off-by: David S. Miller <davem@davemloft.net>