rtc: rs5c372: r2221: fix to use the correct XSTP bit

The Ricoh chips have slightly different register layouts
and the r2221 chip uses bit 5 as the oscillator halt sensor bit.

Signed-off-by: Olive Rohe <oliver.rohe@wago.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>