USB: adjust the Generic XHCI ACPI probe return value

Change the probe return value from BUS_PROBE_DEFAULT to BUS_PROBE_GENERIC
given this is the "generic" attach method. This allows individual
drivers using XHCI generic but needing their own intialisation to
gain priority for attaching over the generic implementation.

Reviewed by: hselasky
Differential Revision: https://reviews.freebsd.org/D32257