802.15.4: fix some compiler warnings.

802.15.4: fix some compiler warnings.

C's type conversion rules are a barrel of fun.  Did you know that if you
add "2" to an "unsigned short", the result has the type "int", even
though, at least on a machine where "unsigned int" is longer than
"unsigned short", the result is always >= 0?

Add "2U" instead, so that one of the operands is an "unsigned int",
making the result an "unsigned int".