rtc: stm32: fix comparison warnings

rtc: stm32: fix comparison warnings

This patches fixes comparison between signed and unsigned values as it
could produce an incorrect result when the signed value is converted to
unsigned:

drivers/rtc/rtc-stm32.c: In function 'stm32_rtc_valid_alrm':
drivers/rtc/rtc-stm32.c:404:21: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  if ((((tm->tm_year > cur_year) &&
...

It also fixes comparison always true or false due to the fact that unsigned
value is compared against zero with >= or <:

drivers/rtc/rtc-stm32.c: In function 'stm32_rtc_init':
drivers/rtc/rtc-stm32.c:514:35: warning: comparison of unsigned expression >= 0 is always true [-Wtype-limits]
  for (pred_a = pred_a_max; pred_a >= 0; pred_a-- ) {

drivers/rtc/rtc-stm32.c:530:44: warning: comparison of unsigned expression < 0 is always false [-Wtype-limits]
     (rate - ((pred_a + 1) * (pred_s + 1)) < 0) ?

Fixes: 4e64350f42e2 ("rtc: add STM32 RTC driver")
Signed-off-by: Amelie Delaunay <amelie.delaunay@st.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>