sys/net: Fix unaligned access on Samr21/Cortex-M0

*((uint16_t)*dest) results in a ldrh (load halfword) instruction
on an address that seems to not be halfword-aligned (?),
causing a hard-fault on the samr21-xpro board (cortex-m0
architecture). The issue seems to be very similar to
the one described in http://stackoverflow.com/a/21661366/124257