rtc: pcf8523: Fix oscillator stop bit handling reading from Control_1

The check if the oscillator stop bit is set was reading from Control_1
register instead of the Seconds register.
This caused the Seconds register to be incorrectly changed if bit 7 of
Control_1 happens to be set.

Signed-off-by: Axel Hammarberg <axel.hammarberg@gmail.com>