net/mqtt: Make input arguments const

This commit changes some function signatures found at the private
MQTT Packet Lib. Input arguments representing arrays can be considered
'const', avoiding compiler warnings when passing "const arrays" in
MQTT applications.

Change-Id: I3ebaa9c7ff5fba74324bf79c55027cdd82669eaa
Signed-off-by: Flavio Santes <flavio.santes@intel.com>