acpi: Fix error code returned in acpi_bus_get_prop

ACPI implementation of device_get_property would return "-1" when
property was found, but it's type wasn't supported.
This causes device_has_property to return false in that scenario, which
arguably could be considered as incorrect.

Fix that by returning "0" in that case.

Reviewed by: bz, mw
Tested by: mw
MFC after: 2 weeks
Obtained from: Semihalf
Differential Revision: https://reviews.freebsd.org/D33103