driver/counter: rtc_stm32: Incorrect time_t offset

When converted to time_t, RTC init date was missing 100 years
offset, as time_t starts in 1900 while RTC starts in 2000.
Besides, tm_mon calculation was wrong by 1 month as allowed range
is 0-11 and provided in range 1-12.

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>