Silicon/Broadcom/Bcm27xx: Relax PCIe device restriction

The CM4 has an actual PCIe slot, so the device filtering
need to be a little less restrictive WRT busses with more
than 1 device given that switches can now appear in the
topology. Since it is possible to start numbering the
busses with a non-zero value, the bus restriction should
be based on the secondary side of the root port. This
isn't likely but its better than hard-coding the limit.

Suggested-by: René Treffer <treffer+groups.io@measite.de>
Signed-off-by: Jeremy Linton <jeremy.linton@arm.com>
Reviewed-by: Andrei Warkentin <awarkentin@vmware.com>
Tested-by: Jared McNeill <jmcneill@invisible.ca>