Current bxe probe function won't attach to devices with the NetXtreme II
BCM57840 2x20GbE chip, enable it by adding it's chip ID to the list of
supported chips.
Tested on: HP ProLiant WS460c Gen9
Reviewed by: gnn
MFC after: 1 week
Sponsored by: Citrix Systems R&D
Differential Revision: https://reviews.freebsd.org/D9609