cellRtc: tick conversion improvements

[capriots]

• Both date_time_to_tick() and tick_to_date_time() now have constant run time
• date_time_to_tick():
  • Replaced with implementation from cellRtcGetTick() which already is exactly what LLE does
  • Instead of counting the days that have passed in previous months in a loop, it now uses a lookup table since those values are always the same for each month
• tick_to_date_time():
  • Microseconds, seconds, minutes, and hours are already calculated the same way as LLE
  • Day, month, and year are calculated differently from LLE but produces the same result for every input and is less complex
  • The amount of days that fit into a u64 tick is small enough that I was able to test this for every single possible day
• Fixed the null pointer check in cellRtcGetTick(). LLE actually returns CELL_OK here.
• Set log level for cellRtcGetTick() and cellRtcSetTick() to notice as these functions are fully implemented

[Megamouse]

Otherwise it seems to be fine, based on some quick tests compared to old behaviour.