[SWSS:portsorch] fix child_ports checking in addLagMember and removeLagMember for strip tag

[baorliu]

What I did
Added child_ports check in addLagMember and removeLagMember for strip tag

Why I did it
portorch sets LAG member's strip tag when adding subport:

    // Change hostif vlan tag for the parent port only when a first subport is created
    if (parentPort.m_child_ports.empty())
    {
        if (!setHostIntfsStripTag(parentPort, SAI_HOSTIF_VLAN_TAG_KEEP))

but if a new member is added later, in addLagMember function, it does not handle strip tag anymore. Cause the new added lag member has wrong tag mode.

How I verified it
without this fix, I observed that addLagMember happened after addSubPort, the strip tag was not set to the LAG members.

~$ grep PortChannel01 h
2024 Sep 27 19:20:33.184213 sfd-t2-lc2 INFO kernel: [394515.489800] PortChannel01: Port device Ethernet-BP232 added
2024 Sep 27 19:20:33.280190 sfd-t2-lc2 INFO kernel: [394515.586016] PortChannel01: Port device Ethernet-BP236 added
2024 Sep 27 19:20:37.040919 sfd-t2-lc2 WARNING swss0#orchagent: :- addSubPort: DEBUG: set SAI_HOSTIF_VLAN_TAG_ORIGINAL for hostif of port PortChannel01
2024 Sep 27 19:20:51.310876 sfd-t2-lc2 NOTICE swss0#orchagent: :- addLagMember: Add member Ethernet-BP232 to LAG PortChannel01 lid:2000000000688 pid:1000000000028
2024 Sep 27 19:20:51.322898 sfd-t2-lc2 NOTICE swss0#orchagent: :- addLagMember: Add member Ethernet-BP236 to LAG PortChannel01 lid:2000000000688 pid:1000000000029

~$ grep PortChannel17 h
2024 Sep 27 19:20:33.160208 sfd-t2-lc2 INFO kernel: [394515.465774] PortChannel17: Port device Ethernet-BP496 added
2024 Sep 27 19:20:33.196199 sfd-t2-lc2 INFO kernel: [394515.504327] PortChannel17: Port device Ethernet-BP500 added
2024 Sep 27 19:20:36.855791 sfd-t2-lc2 WARNING swss1#orchagent: :- addSubPort: DEBUG: set SAI_HOSTIF_VLAN_TAG_ORIGINAL for hostif of port PortChannel17
2024 Sep 27 19:20:51.206028 sfd-t2-lc2 NOTICE swss1#orchagent: :- addLagMember: Add member Ethernet-BP496 to LAG PortChannel17 lid:200000000068a pid:100000000002a
2024 Sep 27 19:20:51.210702 sfd-t2-lc2 NOTICE swss1#orchagent: :- addLagMember: Add member Ethernet-BP500 to LAG PortChannel17 lid:200000000068a pid:100000000002b
~$  

Build the image with the fix, confirmed that the strip tag is set correctly even the addLagMember called after assSubPort.
(will add a testcase for this before convert it to formal PR)
Details if related