korina: reset resource buffer size to 1536

The new value is the one used in the external patch before and allows at
least a standard MTU of 1500 to be handled correctly. Impact of this
change gets visible when bigger packets are to be received, issuing:
| ping -s 492 <IP>
and bigger payload sized led to 100% packet loss.

Signed-off-by: Phil Sutter <n0-1@freewrt.org>
Acked-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: David S. Miller <davem@davemloft.net>