rtc: s5m: cache device type during probe

platform_get_device_id() is called mulitple times during probe to
retrieve the device type. This makes the code harder to read than
necessary.

Just get the type once, which also trims the lengths of the lines
involved.

Signed-off-by: André Draszik <andre.draszik@linaro.org>
Link: https://lore.kernel.org/r/20250409-s2mpg10-v4-25-d66d5f39b6bf@linaro.org


Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>