Fix update with modern ota files

[manup]

• Fix Ikea Tradfri Repeater update https://github.com/dresden-elektronik/deconz-ota-plugin/issues/37
• Fix loading OTA files of new Ledvance products.
• The problem was fixed by using the original file content for transfers. Prior the content was parsed and rebuild which lost some sub tagged areas of recent OTA files.
• Reduce dependencies on file extensions, test for valid OTA content.
• Cap max data size to 50 to support a wider range of devices and fallback to 40 bytes in error cases. The plain source route capping before didn't work out in some cases.