hw/net/opencores_eth: Allocating Large sized arrays to heap

hw/net/opencores_eth: Allocating Large sized arrays to heap

open_eth_start_xmit has a huge stack usage of 65536 bytes approx.
Moving large arrays to heap to reduce stack usage.

Reduce size of a buffer allocated on stack to 0x600 bytes, which is the
maximal frame length when HUGEN bit is not set in MODER, only allocate
buffer on heap when that is too small. Thus heap is not used in typical
use case.

Signed-off-by: Zhou Jie <zhoujie2011@cn.fujitsu.com>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>