jedec_dimm: Use device_set_descf()

Note that it is ok to use device_get_desc() as one of the format string
parameters because it is set using device_set_desc() (not
device_set_desc_copy()) and so won't be freed when the description is
updated.

No functional change intended.

MFC after: 1 week

(cherry picked from commit 83caa244bc9eef6949a1250a875ad1409775a46e)