net/gianfar: drop recycled skbs on MTU change

The size for skbs which is added to the recycled list is using the
current descriptor size which is current MTU. gfar_new_skb() is also
using this size. So after changing or alteast increasing the MTU all
recycled skbs should be dropped.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Acked-by: Andy Fleming <afleming@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>