UART backend: make FIFO width an option

[imphil]

Currently the UART backend has no way to auto-discover the FIFO width on the hardware side, hence the user of the library must call glip_set_fifo_width() to set it. That's a bit strange, since it's only required for the UART backend. Another solution would be to make the FIFO width an option passed to GLIP like speed, device, etc.