ice: Respond to a NETDEV_UNREGISTER event for LAG

When the PF is a member of a link aggregate, and the driver
is removed, the process will hang unless we respond to the
NETDEV_UNREGISTER event that is sent to the event_handler
for LAG.

Add a case statement for the ice_lag_event_handler to unlink
the PF from the link aggregate.

Also remove code that was incorrectly applying a dev_hold to
peer_netdevs that were associated with the ice driver.

Fixes: df006dd4

 ("ice: Add initial support framework for LAG")
Signed-off-by: Dave Ertman <david.m.ertman@intel.com>
Tested-by: Tony Brelinski <tony.brelinski@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>