iflib: provide probe wrapper for vendor drivers

From Jake:
Vendor drivers that exist out-of-tree generally should return
BUS_PROBE_VENDOR from their device probe functions. This helps ensure
that a vendor replacement driver will supersede the in-kernel driver for
a given device.

Currently, if a vendor wants to implement a driver based on iflib, it
will always report BUS_PROBE_DEFAULT.

Add a wrapper function, iflib_device_probe_vendor() which can be used in
place of iflib_device_probe(). This function will just return
BUS_PROBE_VENDOR whenever iflib_device_probe() would return
BUS_PROBE_DEFAULT.

While vendor drivers can already implement such a wrapper themselves,
providing it in the iflib.h header makes it easier for the vendor driver
to do the right thing.

Submitted by: Jacob Keller <jacob.e.keller@intel.com>
Reviewed by: erj@, gallatin@, marius@
MFC after: 1 week
Sponsored by: Intel Corporation
Differential Revision: https://reviews.freebsd.org/D20221