Do not fail 'devctl clear driver' if another driver is not found

Detaching the bhyve(4) ppt driver from an unsupported PCI device
should not raise a "Device not configured" error. We do not expect
that a new driver must take over the device in this case.

Reviewed by: imp, jhb
Differential Revision: https://reviews.freebsd.org/D52050

(cherry picked from commit fba56be09f1c344cc5805b898fbfef6066950942)