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

(cherry picked from commit 76f3b8cb640536de2c370cc2bd60382bbc35cf5d)