net: lwm2m: setup data_ptr/len for OPAQUE resource when none given

OPAQUE resource type might/might not have data_ptr/data_len setup
depending on the implementation. This introduce an issue that when
OPAQUE resource is written from the server side, the ones w/ none
setup will not be able to get the data at post_write_cb()

Modify to setup data_ptr/data_len as incoming buffer and buffer size

Signed-off-by: Robert Chou <robert.ch.chou@acer.com>