Ponmngr: set vendorId when XPON init in sh script

Link to task:
This change is made for this ticket https://project.iopsys.eu/issues/10089

This change is tied to this MR https://dev.iopsys.eu/feed/airoha/-/merge_requests/87

Short results

Before patch:

root@nvg578m-2c9569ffeeb7:~# /userfs/bin/omcicfgCmd get vendorId
vendorId = MTKG

After patch:

root@nvg578m-2c9569ffeeb7:~# /userfs/bin/omcicfgCmd get vendorId
vendorId = IOPS
root@nvg578m-2c9569ffeeb7:~#