To fix the build after ppp_output_cb started taking it as const in
commit b2d1fc119d.
Fixes this failure:
../contrib/examples/ppp/pppos_example.c: In function ‘ppp_output_cb’:
../contrib/examples/ppp/pppos_example.c:163:29: error: cast discards ‘const’ qualifier from pointer target type [-Werror=cast-qual]
return sio_write(ppp_sio, (u8_t*)data, len);
^
There is no good reason why this function should take a non-const
pointer, as the output callback should never modify what lwIP gives it.
While changing that also switch to a more generic `void*` instead of
"byte".
