net/prestera: Fix devlink groups leakage in error flow

Devlink trap group is registered but not released in error flow,
add the missing devlink_trap_groups_unregister() call.

Fixes: 0a9003f4 ("net: marvell: prestera: devlink: add traps/groups implementation")
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>