net: bandaid for plugging a fw_com leak in fwip_detach()

Adding a temporary workaround for plugging a fw_com upon if_fwip unloading.

Steps to reproduce(needs two hosts connected with firewire):

while true; do
  ifconfig fwip0 10.0.0.5 up
  fwcontrol -r
  ping -c 10.0.0.3
  kldunload if_fwip
done

There's a chance that the unloading of if_fwip.ko triggers following warning:

Warning: memory type fw_com leaked memory on destroy (1 allocations, 64 bytes leaked).

commit d79b6b8ec267e7eef6e07cf4245159705e24acd5 (origin/main, origin/HEAD)