drivers/net: do not touch mbuf next or nb segs on Rx

Now that the m->next pointer and m->nb_segs is expected to be set (to
NULL and 1 respectively) after a mempool_get(), we can avoid to write them
in the Rx functions of drivers.

Only some drivers are patched, it's not an exhaustive patch. It gives
the idea to do the same in other drivers.

Signed-off-by: Olivier Matz <olivier.matz@6wind.com>