net/sixlowpan: Fixed calculation of fragment size

Fragment size calculation previously failed for devices that are able to
transmit bigger layer 2 PDUs that 802.15.4 devices. This commit fixes the issue.

sys/net/gnrc/network_layer/sixlowpan/frag/gnrc_sixlowpan_frag.c

@@ -44,7 +44,7 @@ static uint16_t _tag;
 
 static inline uint16_t _floor8(uint16_t length)
 {
-    return length & 0xf8U;
+    return length & 0xfff8U;
 }
 
 static inline size_t _min(size_t a, size_t b)