Gigaset N510 enhancements

Hello,
This PR enhances Gigaset N510 plugin.
First, it enables firmware auto-update, thanks to properly formatted links provided by Gigaset.
As found in the documentation :
https://teamwork.gigaset.com/gigawiki/display/GPPPO/Direct+link+to+Firmware+file
While here, also update to last firmware version.
https://teamwork.gigaset.com/gigawiki/display/GPPPO/Firmware+N510+IP+PRO
It also properly assign each DECT to its SIP line only.
By default, on N510, all DECT ring / receive calls for all the 6 configured SIP accounts.
And all DECT make outgoing calls from the first configured SIP account.
Which does not make sense...
This PR then properly assign each DECT to its proper SIP account : first DECT to first SIP account, second DECT to second SIP account etc..
Which is then the expected behaviour...
Thank you 👍

Reviewed-by: Alexandre Fournier <None>

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