power: supply: bq25980_charger: Constify reg_default array

Static 'struct reg_default' array is not modified so can be changed to
const for more safety.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20250528194439.567263-4-krzysztof.kozlowski@linaro.org


Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>