6lowpan: change naming for lowpan private data

This patch changes the naming for interface private data for lowpan
intefaces. The current private data scheme is:

-------------------------------------------------
|    6LoWPAN Generic   |    LinkLayer 6LoWPAN   |
-------------------------------------------------

the current naming schemes are:

- 6LoWPAN Generic:
  - lowpan_priv
- LinkLayer 6LoWPAN:
  - BTLE
    - lowpan_dev
  - 802.15.4:
    - lowpan_dev_info

the new naming scheme with this patch will be:

- 6LoWPAN Generic:
  - lowpan_dev
- LinkLayer 6LoWPAN:
  - BTLE
    - lowpan_btle_dev
  - 802.15.4:
    - lowpan_802154_dev

Signed-off-by: Alexander Aring <aar@pengutronix.de>
Reviewed-by: Stefan <Schmidt&lt;stefan@osg.samsung.com>
Acked-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>