LinuxKPI: MODULE_DEVICE_TABLE() remove hard coded pci

While we currently only use MODULE_DEVICE_TABLE() for pci (or so I
thought [*]), this may soon change. Remove a hard coded "pci" for
the bus and use the bus name passed in as _bus.

• see follow-up change

Sponsored by: The FreeBSD Foundation
Reviewed by: imp, dumbbell
Differential Revision: https://reviews.freebsd.org/D51048

(cherry picked from commit f58e032410c93d67c83d6bce8a3e6711f377a4be)