bhndb(4): Fix probing of bhndb-attached bhnd_nvram devices.

This fixes bhnd(4) nvram handling on devices that map SPROM CSRs via PCI
configuration space.

The probe method previously required that a bhnd(4) device be attached to the
parent bridge; now that the bhnd_nvram device is always attached first, this
unnecessary sanity check always failed.

Approved by: adrian (mentor, implicit)