TSCH: fix a wrong return value
TSCH: fix a wrong return value

tsch_security_secure_frame() returns the length of MIC in case of
success. Otherwise, it is supposed to return 0. There was one case where
it returns 1 in an error handling block.

arurke/contiki,arurke/contiki,MohamedSeliem/contiki,MohamedSeliem/contiki,arurke/contiki,MohamedSeliem/contiki,MohamedSeliem/contiki,arurke/contiki,arurke/contiki,MohamedSeliem/contiki,MohamedSeliem/contiki,arurke/contiki,arurke/contiki,MohamedSeliem/contiki