Fix serialdump.c warnings (clang)
Fix serialdump.c warnings (clang)

Using clang (on osx) yields to some warnings
(conversion between signed/unsigned, unused variable)
which gcc would only produce when used with -Wall.
This commit fixes those.
arurke/contiki,MohamedSeliem/contiki,MohamedSeliem/contiki,arurke/contiki,arurke/contiki,MohamedSeliem/contiki,MohamedSeliem/contiki,arurke/contiki,MohamedSeliem/contiki,arurke/contiki,MohamedSeliem/contiki,arurke/contiki,MohamedSeliem/contiki,arurke/contiki