lantiq: net: fix duplicated skb in rx descriptor ring

The previous commit didn't fix the bug properly. By mistake, it replaces
the pointer of the next skb in the descriptor ring instead of the current
one. As a result, the two descriptors are assigned the same SKB. The error
is seen during the iperf test when skb_put tries to insert a second packet
and exceeds the available buffer.

Fixes: c7718ee9

 ("net: lantiq: fix memory corruption in RX ring ")
Signed-off-by: Aleksander Jan Bajkowski <olek2@wp.pl>
Signed-off-by: David S. Miller <davem@davemloft.net>