[bhnd] Fix ChipCommon probing.

ChipCommon probing uses mapping table "chipc_devices". It calls bhnd_device_lookup,
which iterate over mapping table with end condition:

entry->desc != NULL

So if mapping table contains row with description equals to NULL, it will
stop processing of mapping. I.e. description is mandatory field and should
be not NULL.

This patch corrects mapping table for ChipCommon.

Submitted by: Michael Zhilin <mizhka@gmail.com>
Differential Revision: https://reviews.freebsd.org/D6088