rtc: sd2405al: drop needless struct sd2405al::rtc member

rtc: sd2405al: drop needless struct sd2405al::rtc member

The memory pointed to by the ::rtc member is managed via devres, and
no code in this driver uses it past _probe().

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

Tested-by: Tóth János <gomba007@gmail.com>
Signed-off-by: André Draszik <andre.draszik@linaro.org>
Link: https://lore.kernel.org/r/20250304-rtc-cleanups-v2-13-d4689a71668c@linaro.org
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>