r351152 introduced iflib_deregister() function calling
EVENTHANDLER_DEREGISTER() to unregister VLAN events. This patch removes
duplicate of EVENTHANDLER_DEREGISTER() calls placed in
iflib_device_deregister() as this function is now calling
iflib_deregister(). This is to avoid deregistering same event twice.

This patch also adds check in iflib_vlan_register() to prevent
registering VLAN while being in detach.

Patch co-authored by Krzysztof Galazka <krzysztof.galazka@intel.com>,
erj <erj@FreeBSD.org> and Jacob Keller <jacob.e.keller@intel.com>.

Signed-off-by: Piotr Pietruszewski <piotr.pietruszewski@intel.com>

Before applying this patch, kernel panic should be observed after issuing:

for i in {1..10}; do kldunload if_ix; kldload if_ix; done

Usually, less than 10 cycles are needed to trigger the panic. After applying
the patch, panic should be gone.