ofw: remove redundant calls in ofwbus_attach()

Since commit ecaecbc7d8bc212d8e854088106b3b21e631bb52, calling
ofw_bus_gen_setup_devinfo() is redundant, as the call to
device_set_ivars() now happens inside simplebus_add_device().

Reviewed by: markj
Approved by: markj (mentor)
Differential Revision: https://reviews.freebsd.org/D40271