update handle_fn

[f4grx]

according to the goodfet documentation, the "length" packet of the goodfet protocol frames is 16 bits long.

however the handler functions use a 32 bits integer to hold the length, as in goodfet.c :

void handle(uint8_t const app,
        uint8_t const verb,
        uint32_t const len)

Is there any reason for that? I propose to change all handler functions to use uint16_t instead, but I would like to understand the issue here not to break anything.

[travisgoodspeed]

Thanks for you help with cleaning this code up, but the handler function should remain uint32_t to support future expansion, such as devices that do USB directly.

--Travis