sys/transceiver: fix t9r_register API compliance
sys/transceiver: implement transceiver_unregister
sys/transceiver: refactor un/register

remove race conditions
make less restrictive (allow double-un/registers)
make loops more readable
sys/transceiver: use size_t instead of u8 in loops
Merge pull request #1395 from LudwigOrtmann/transceiver_unregister

sys/transceiver: add transceiver_unregister and refactor transceiver_register