netsender: change warning log level string from "Warn" to "Warning" for consistency

The warning log level string is defined in multiple places in our repositories. It’s in AV av/config/variables.go ln: 378, and in iot pi/netsender/netsender.go ln: 870.

Both are checked against when the log level for a device is set to Warning. This means it is impossible to set the log level to warning without creating a configuration issue.

We should change the string from ‘Warn’ to ‘Warning’ in netsender.go.