rtc: pcf8563: fix output clock rate

The pcf8563_clkout_recalc_rate function erroneously ignores the
frequency index read from the CLKO register and always returns
32768 Hz.

Fixes: a39a6405

 ("rtc: pcf8563: add CLKOUT to common clock framework")
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>