gianfar: Remove superfluous kernel_dropped local counter

The GRO_DROP return code is handled by the core network layer.
The current kernel approach is to factorize this kind of statistics into
the upper layers, instead of having all the drivers maintaining them.

Signed-off-by: Claudiu Manoil <claudiu.manoil@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>