hidpp20drv: rename the internal per feature read_button
hidpp20drv: rename the internal per feature read_button

Allows to navigate in the source more easily if they share a common
name.
However, this is still not good because we need to know which feature
number is which.

Though the format is now in sync with write_button.

Signed-off-by: Benjamin Tissoires <7a0e50e3f6a0939db82b6a8a423c08a163896f41@gmail.com>

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