rtc: pcf85063: preserve control register value between stop and start

Fix a bug that caused the Control_1 register to get zeroed whenever the
RTC time is set. The problem occurred between stopping and starting the
RTC clock, wherein the return value of a successful I2C write function
would get written to the register.

Also update variables of the start and stop functions to be more
consistent with the rest of the driver.

Signed-off-by: Alvin Šipraga <alvin@airtame.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>