rtc: ds1307: fix RX8130 wakeup alarm WADA bit for day-of-month mode

The RX8130 wakeup alarm never fired when set via
/sys/class/rtc/rtc0/wakealarm. The root cause is that the WADA bit
(bit 3) in the Extension register (0x1c) was never set before
programming the alarm registers.

Per the RX8130 datasheet:
WADA=0 - Week alarm: register 0x19 is compared against day-of-week
WADA=1 - Day alarm:  register 0x19 is compared against day-of-month

rx8130_set_alarm() always writes a BCD day-of-month value to alarm
register 0x19, so WADA must be 1. With WADA=0 the hardware matched the
day-of-month value (e.g. 15) as a day-of-week index, which is always
out of range (valid weekdays are 0-6), so the alarm interrupt was never
asserted.

Fix by setting the WADA bit in rx8130_set_alarm() before writing the
Extension register back to the device.

This is consistent with rx8130_read_alarm(), which decodes the same
register under the same assumption (24-hour and day-of-month mode).

Signed-off-by: Robert Leussler <robert.leussler@leica-geosystems.com>
Link: https://patch.msgid.link/20260721081907.3518648-1-robert.leussler@leica-geosystems.com
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

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