samr21-xpro: unable to ping another node

[thomaseichinger]

git bisect hints to #7115 having introduced an regression, pinging between two samr21-xpro nodes doesn't work on current master as of 524f2e5.

@aabadie can you confirm?

[miri64]

Can confirm for iotlab-m3 and samr21-xpro (did not check when the regression was introduced though)

[miri64]

For some reason it already fragments 38 byte packets with lots of duplication oO.

[miri64]

Of course, when I add debug output it works -.-