bhyve: remove unnecessary const qualifier in acpi_device.h

Those const qualifier declare that the function doesn't change the
values internally. It makes no sense to add them in the header file.

Reviewed by: markj
MFC after: 1 week
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D39318

(cherry picked from commit 404689908632599ee5b3bcbc289ae7f094368dc5)