Remove casting the values returned by memory allocation function.

Coccinelle emits WARNING:

./drivers/net/ethernet/hisilicon/hix5hd2_gmac.c:1027:9-23: WARNING:
 casting value returned by memory allocation function to (struct sg_desc *) is useless.

This issue was detected by using the Coccinelle software.
Signed-off-by: Wang Hai <wanghai38@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>