rtc: st-lpc: Add missing clk_disable_unprepare in st_rtc_probe()

The clk_disable_unprepare() should be called in the error handling
of clk_get_rate(), fix it.

Fixes: b5b2bdfc

 ("rtc: st: Add new driver for ST's LPC RTC")
Signed-off-by: Gaosheng Cui <cuigaosheng1@huawei.com>
Link: https://lore.kernel.org/r/20221123014805.1993052-1-cuigaosheng1@huawei.com
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>