drivers/wireless/lpwan/rn2xx3: Patch read operation

[linguini1]

Summary

This patch fixes an issue with the read operation where sometimes the file_read call would return a single ASCII hex byte instead of 2 and the parsing logic could not handle that scenario. Now packet data is read in large chunks to an internal buffer and incrementally parsed, making it robust to this problem (and likely much faster).

Closes #17273.

Impact

The driver implementation should no longer have this read bug, and read operations should be faster. This is a change to the internal implementation, doesn't touch documentation or build system.

Testing

Only verified that the code compiles without warnings/errors using the raspberrypi-pico:usbnsh configuration with the driver enabled.

Waiting for fix confirmation from @bskdany after device testing, marked as draft until then.