drivers: counter: stm32: fix rtc subsecond register read

According to the reference manual, 'When the BYPSHAD control bit is set
in the RTC_CR register [...] the value of one of the registers may be
incorrect if an RTCCLK edge occurs during the read operation'. We need
to read te subseconds register until two successive reads are equal.

Signed-off-by: Adrien Lessard <adrien.lessard.42@gmail.com>