nvmem: specify ->reg_read/reg_write() expected return values

Both ->reg_read() and ->reg_write() return values are not easy to
deduce. Explicit that they should return zero on success (and negative
values otherwise).

Such callbacks, in some alternative world, could return the number of
bytes in the success case. That would be translated to errors in the
nvmem core because of checks like:

	ret = nvmem->reg_write(nvmem->priv, offset, val, bytes);
	if (ret) {
		// error case
	}

This mistake is not just theoretical, see commit
28b008751aa2 ("nvmem: rmem: Fix return value of rmem_read()").

Signed-off-by: Théo Lebrun <theo.lebrun@bootlin.com>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Link: https://lore.kernel.org/r/20241230143035.265518-4-srinivas.kandagatla@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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