bhndb_pci(4): Implement bridge support for CardBus-attached devices.

• Extend the probe method to accept devclasses that inherit from the pci devclass (e.g. cardbus).
• Some BCM4306-based CardBus adapters appear to advertise 4K SPROM, but only the first 2K is mapped into BAR0. We can safely assume that the SPROM data fits within the first 2K of the SPROM, rather than rejecting the SPROM mapping as invalid.

Sponsored by: The FreeBSD Foundation