rtc: ds1307: improve trickle charger initialization

Instead of storing the trickle_charger_setup value in struct chip_desc
we can let function ds1307_trickle_init return it because it's used
in the probe function only.
This allows us to constify struct chip_desc variables in a next step.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>