Configuring RFO driver resistance for ST25R39

I'm looking to make the RFO driver resistance for the ST35R39 configurable, because it looks like we need a different value for our project, but I have a question about the approach.

Currently it is set to a fixed value in the [mode_on()](https://github.com/embassy-rs/rnfc/blob/7891744b9aa5b820e2c48f98b43e4163beb9d34f/rnfc-st25r39/src/lib.rs#L440) function. This means the the value is set every time card polling is started. Is there a specific reason to do this every time, or could it be simplified to set it once during initialization?

The latter case would allow for a slightly cleaner way to make this configurable. I can't find anything in the documentation to suggest it needs to be set every time, but if anyone knows this do be different I'd like to hear that before breaking things. If it's just there for historical reasons it could try to do some testing to determine of it ever gets changed.

Start in rnfc-st25r39/src/lib.rs at mode_on(), where the RFO driver resistance is currently set during card polling. First determine through the ST25R39 documentation and project-hardware testing whether the setting must be repeated; done means establishing that behavior and agreeing on a configurable initialization or polling-time approach.

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