net: hns3: refine function hclge_set_vf_vlan_cfg()

The struct hclge_vf_vlan_cfg is firstly designed for setting
VLAN filter tag. And it's reused for enable RX VLAN offload
later. It's strange to use member "is_kill" to indicate "enable".
So redefine the struct hclge_vf_vlan_cfg to adapt it.

For there are already 3 subcodes being used in function
hclge_set_vf_vlan_cfg(), use "switch-case" style for each
branch, rather than "if-else". Also simplify the assignment for
each branch to make it more clearly.

Signed-off-by: Jian Shen <shenjian15@huawei.com>
Signed-off-by: Huazhong Tan <tanhuazhong@huawei.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>