luci-proto-ncm: rename `service` option to `mode`

Package luci-proto-ncm depends on comgt-ncm which uses an option
called 'mode' to set the radiomode of the modem. There is no option
'service' in the comgt-ncm scripts.

Suggested-by: breenstorm <49235337+breenstorm@users.noreply.github.com>
[fix commit subject, add commit message, rebase onto master branch]
Signed-off-by: Jo-Philipp Wich <jo@mein.io>