6lowpan: Change byte order when storing/accessing u16 tag

The tag field should be stored and accessed using big endian byte order (as
intended in the specs). Or else, when displayed with a trafic analyser, such a
Wireshark, the field not properly displayed (e.g. 0x01 00 instead of 0x00 01,
and so on).

Signed-off-by: Tony Cheneau <tony.cheneau@amnesiak.org>
Signed-off-by: David S. Miller <davem@davemloft.net>