rtc: max77686: drop needless struct max77686_rtc_info::rtc member

rtc: max77686: drop needless struct max77686_rtc_info::rtc member

When this driver was converted to using the devres managed i2c device
in commit 59a7f24fceb3 ("rtc: max77686: convert to
devm_i2c_new_dummy_device()"), struct max77686_rtc_info::rtc became
essentially unused.

We can drop it from the structure and just use a local temporary
variable, reducing runtime memory consumption by a few bytes.

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: André Draszik <andre.draszik@linaro.org>
Link: https://lore.kernel.org/r/20250304-rtc-cleanups-v2-1-d4689a71668c@linaro.org
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>