rtc: ftrtc010: drop needless struct ftrtc010_rtc::rtc_dev member

rtc: ftrtc010: drop needless struct ftrtc010_rtc::rtc_dev member

The memory pointed to by the ::rtc_dev 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.

Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: André Draszik <andre.draszik@linaro.org>
Link: https://lore.kernel.org/r/20250304-rtc-cleanups-v2-6-d4689a71668c@linaro.org
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>