net: hns3: optimize function hclge_cfg_common_loopback()

hclge_cfg_common_loopback() is a bit too long, so
encapsulate hclge_cfg_common_loopback_cmd_send() and
hclge_cfg_common_loopback_wait() two functions to
improve readability.

Signed-off-by: Yufeng Mo <moyufeng@huawei.com>
Signed-off-by: Guangbin Huang <huangguangbin2@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>