drivers: net: xgene: fix IPv4 forward crash

This patch fixes the crash observed during IPv4 forward test by
setting the drop field in the dbptr.

Signed-off-by: Iyappan Subramanian <isubramanian@apm.com>
Tested-by: Toan Le <toanle@apm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>