slipdev: simplify and solidify byte-unstuffing

This simplifies and solidifies the reversal of SLIP's byte-stuffing
(aka byte-unstuffing ;-)) by

1. Using `tsrb` instead of `ringbuffer`: there are two actors. The ISR
   and the device's event handler.
2. Moving the byte-unstuffing from the UART RX-handler (i.e. the ISR)
   to the device's receive function (potentially not the ISR)
3. Removing the `pktfifo` member. The current number of bytes in the
   ringbuffer is returned for `recv(data = NULL, len = 0)`. If that is
   more than the packet contains (due to the byte stuffing it most
   likely will be) the packet is reallocated in GNRC anyway.