rtc: mt6359: Use RTC_TC_DOW hardware register for wday

rtc: mt6359: Use RTC_TC_DOW hardware register for wday

Instead of calculating the number of full days since Sunday with
(days + 4) % 7, read (and write) that to the RTC Day-of-week Time
Counter register (RTC_TC_DOW).

Some transformation (addition and subtraction for set/get) is
still done, as this register's range is [1..7], while the tm_wday
in struct tm's range is [0..6].

Please note that this was added only to set_time() and read_time()
callbacks because set_alarm() and read_alarm() are setting a bit
in RTC_AL_MASK to ignore DOW for RTC HW alarms for unknown reasons.

Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://lore.kernel.org/r/20240923100010.97470-4-angelogioacchino.delregno@collabora.com
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>