Ieee802154: use enums for frame type

Typing the frame type and replace some literals.
Yes, `sizeof(ieee802154_frame_type)` can be `1`, `2` or `4`, depending on the ABI.
With `__attribute__((packed))` it will be 1, because a char can take all values in 0..3.

Or doesn't the struct `ieee802154_frame_fcf_frame_t` not need to fit some predefined format?