gnrc/netif: 6lo: use 16 bit for max_frag_size
gnrc/netif: 6lo: use 16 bit for max_frag_size

Using 8 bit for `max_frag_size` limits that value to 255.
802.15.4g specifies a PDU of 2047 bytes which exceeds that limit.

Using a 16 bit value here allows to use the full L2 PDU.

Before:

    12:02:16,300 # ping6 fe80::2068:3123:59f5:d238%8 -s 400
    12:02:16,302 # sending 244 bytes
    12:02:16,387 # sending 218 bytes
    12:02:16,624 # 408 bytes from fe80::2068:3123:59f5:d238%8: icmp_seq=0 ttl=64 rssi=-49 dBm time=316.307 ms
    12:02:17,302 # sending 244 bytes
    12:02:17,387 # sending 218 bytes
    12:02:17,624 # 408 bytes from fe80::2068:3123:59f5:d238%8: icmp_seq=1 ttl=64 rssi=-49 dBm time=316.307 ms
    12:02:18,302 # sending 244 bytes
    12:02:18,387 # sending 218 bytes
    12:02:18,624 # 408 bytes from fe80::2068:3123:59f5:d238%8: icmp_seq=2 ttl=64 rssi=-50 dBm time=316.306 ms
    12:02:18,625 #
    12:02:18,629 # --- fe80::2068:3123:59f5:d238 PING statistics ---

With this patch:

    12:09:44,276 #  ping6 fe80::2068:3123:59f5:d238%8 -s 400
    12:09:44,278 # sending 432 bytes
    12:09:44,574 # 408 bytes from fe80::2068:3123:59f5:d238%8: icmp_seq=0 ttl=64 rssi=-52 dBm time=289.888 ms
    12:09:45,279 # sending 432 bytes
    12:09:45,574 # 408 bytes from fe80::2068:3123:59f5:d238%8: icmp_seq=1 ttl=64 rssi=-52 dBm time=289.885 ms
    12:09:46,069 # sending 43 bytes
    12:09:46,279 # sending 432 bytes
    12:09:46,499 # sending 43 bytes
    12:09:46,574 # 408 bytes from fe80::2068:3123:59f5:d238%8: icmp_seq=2 ttl=64 rssi=-47 dBm time=289.886 ms
    12:09:46,574 #
    12:09:46,578 # --- fe80::2068:3123:59f5:d238 PING statistics ---

RIOT-OS/RIOT,RIOT-OS/RIOT,OlegHahm/RIOT,ant9000/RIOT,OlegHahm/RIOT,authmillenon/RIOT,kaspar030/RIOT,basilfx/RIOT,kYc0o/RIOT,OTAkeys/RIOT,basilfx/RIOT,jasonatran/RIOT,basilfx/RIOT,miri64/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,ant9000/RIOT,authmillenon/RIOT,jasonatran/RIOT,OTAkeys/RIOT,jasonatran/RIOT,kaspar030/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,miri64/RIOT,kaspar030/RIOT,kYc0o/RIOT,basilfx/RIOT,OTAkeys/RIOT,OTAkeys/RIOT,authmillenon/RIOT,kYc0o/RIOT,ant9000/RIOT,OTAkeys/RIOT,kYc0o/RIOT,OlegHahm/RIOT,kYc0o/RIOT,ant9000/RIOT,OlegHahm/RIOT,miri64/RIOT,OlegHahm/RIOT,ant9000/RIOT,jasonatran/RIOT,authmillenon/RIOT,basilfx/RIOT,miri64/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,authmillenon/RIOT,miri64/RIOT,jasonatran/RIOT