net: hns3: Remove tx ring BD len register in hns3_enet

There is no HNS3_RING_TX_RING_BD_LEN_REG register according
to UM, so this patch removes it.

Signed-off-by: Yunsheng Lin <linyunsheng@huawei.com>
Signed-off-by: Peng Li <lipeng321@huawei.com>
Signed-off-by: Salil Mehta <salil.mehta@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>