net/net: Unregister net_set_vlan command on unload

net/net: Unregister net_set_vlan command on unload

The commit 954c48b9c (net/net: Add net_set_vlan command) added command
net_set_vlan to the net module. Unfortunately the commit only added the
grub_register_command() call on module load but missed the
grub_unregister_command() on unload. Let's fix this.

Fixes: CVE-2025-54770
Fixes: 954c48b9c (net/net: Add net_set_vlan command)
Reported-by: Thomas Frauendorfer | Miray Software <tf@miray.de>
Signed-off-by: Thomas Frauendorfer | Miray Software <tf@miray.de>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>