ppp_generic: use stats from net_device structure

Use stats which now is in the net_device instead of one declared in
ppp structure.
Kill ppp_net_stats function, because by default it is used identical
internal_stats function from net/core/dev.c

Signed-of-by: Paulius Zaleckas <paulius.zaleckas@teltonika.lt>
Signed-off-by: David S. Miller <davem@davemloft.net>