sfxge(4): add API to inform libefx of hardware removal

The efx_nic_hw_unavailable() checks ensure that if the NIC hardware
has failed or has been physically removed then libefx will stop
further attempts to access the hardware.

Add an interface for libefx clients to force unavailability, so the
hardware is treated as dead or removed even if still physically present.

Submitted by: Andy Moreton <amoreton at solarflare.com>
Sponsored by: Solarflare Communications, Inc.
Differential Revision: https://reviews.freebsd.org/D18261