Add parameters on iOS to fix problems with DFU

[dagyu]

This pull request adds the parameters needful to fix the problem described here https://github.com/NordicSemiconductor/IOS-Pods-DFU-Library/issues/368#issuecomment-619066196.

So if you have some kind of troubles with DFU on iOS you can follow the solution explained here https://github.com/NordicSemiconductor/Android-DFU-Library/issues/262#issuecomment-665493850 and then use the parameter forceScanningForNewAddressInLegacyDfu in a way like that:

FlutterNordicDfu.startDfu(
   //other parameters
   iosSpecialParameter: IosSpecialParameter(forceScanningForNewAddressInLegacyDfu: true),
)

The changes made are:

• add forceScanningForNewAddressInLegacyDfu in IosSpecialParameter but this should be done in the Android version too
• mapped the parameters enablePRNs and numberOfPackets on the iOS version because until now they aren't used
• removed a duplicate parameter