rtc: rtc-ds2404: constify ds2404_chip_ops structures

The ds2404_chip_ops structure is never modified, so declare it as const.

Done with the help of Coccinelle.

Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>