rtc: ds1307: remove flags

flags is now unused, drop it.

Tested-by: Łukasz Stelmach <l.stelmach@samsung.com>
Reviewed-by: Łukasz Stelmach <l.stelmach@samsung.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20210418000023.995758-2-alexandre.belloni@bootlin.com