LinuxKPI: netdevice: add alloc_netdev_dummy()

Add alloc_netdev_dummy() which is needed by an updated rtw88 driver.
Given we already call linuxkpi_init_dummy_netdev() ourselves allow the
setup function to be NULL to not initialize and start a tasq td twice.

Sponsored by: The FreeBSD Foundation
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D46802