rtc: rs5c372: r2025: fix check for 'oscillator halted' condition

The R2025SD chip, according to its data sheet, sets the /XST
bit to zero if the oscillator stops. Hence the check for this
condition was wrong.

Signed-off-by: Thomas Koeller <thomas.koeller@baslerweb.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>