powerpc psim: Fix infinite recursion in bus_adjust_resource method

The default case needs to call bus_generic_adjust_resource to pass the
request up the tree, not bus_adjust_resource which will just call this
method again.

Fixes: d7c16b333455 powerpc psim: Use bus_generic_rman_*
(cherry picked from commit ccb1b43e20f5a1a0795b14f3ef39d5c2e15b424d)