rtc: st-lpc: Constify st_rtc_ops

The only usage of st_rtc_ops is to assign its address to the ops field
in the rtc_device struct. which is a const pointer. Make it const to
allow the compiler to put it in read-only memory.

Signed-off-by: Rikard Falkeborn <rikard.falkeborn@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20200913122644.35515-1-rikard.falkeborn@gmail.com