Although we do not yet enable multiple PFs per port, it is possible
that a board will be reconfigured to enable them while the driver has
not yet been updated to fully support this.

The most obvious problem is that multiple functions may try to set
conflicting link settings.  But we will also run into trouble if the
firmware doesn't consider us fully trusted.  So, abort probing unless
both the LinkCtrl and Trusted flags are set for this function.
Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>