net/ieee802154: Make RAW mode generic

- Renaming NET_L2_RAW_CHANNEL to NET_RAW_MODE
- Create a generic IEEE 802.15.4 raw mode for drivers
- Modify the IEEE 802.15.4 drivers so it passes the packet unmodified,
up to code using that mode to apply the necessary changes on the
received net_pkt according to their needs
- Modify wpanusb/wpan_serial relevantly

Fixes #5004

Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>