acpi: Use bus_generic_alloc_resource instead of duplicating it

No functional change, but it is cleaner to use the existing generic
wrappers rather than KOBJ methods directly.

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D43686

(cherry picked from commit 1fb5445206a5ef2351c32f8bc6beb67605593caa)