net: hns3: split function hns3_set_l2l3l4()

Function hns3_set_l2l3l4() is a bit too long. So add two
new functions hns3_set_l3_type() and hns3_set_l4_csum_length()
to simplify code and improve code 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>