luci-app-ddns: fix Event Network value source

[findlayfeng]

Event Network value source should be NetworkSelect instead of DeviceSelect

• [x] This PR is not from my main or master branch :poop:, but a separate branch :white_check_mark:
• [x] Each commit has a valid :black_nib: Signed-off-by: <[email protected]> row (via git commit --signoff)
• [x] Each commit and PR title has a valid :memo: <package name>: title first line subject for packages
• [ ] Incremented :up: any PKG_VERSION in the Makefile
• [x] Tested on: (arm/r5c, main, vivaldi) :white_check_mark:
• [x] ( Preferred ) Mention: Ansuel Smith [email protected]
• [ ] ( Preferred ) Screenshot or mp4 of changes:
• [ ] ( Optional ) Closes: e.g. openwrt/luci#issue-number
• [x] ( Optional ) Depends on:
• [x] Description: The source of network events is the interface rather than the device