rtc: bq32k: Fix handling of oscillator failure flag

While the oscillator failure flag is set, the RTC registers
should be considered invalid. bq32k_rtc_read_time() now
returns an error instead of an invalid time.

The failure flag is cleared the next time the clock is set.

Signed-off-by: Jan Östlund <jao@hms.se>
Signed-off-by: Daniel Romell <daro@hms.se>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>