Ieee802154: use enums for frame type

Typing the frame type and replace some literals.
I'd interpret "The expression that defines the value of an enumeration constant shall be an integer
constant expression that has a value representable as an int." as enum must fit in int. That's from the standard (to be precise: from the draft of the c99 standard, page 105 (117 in pdf) - http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1256.pdf).