rtc: ds1390: fix number of bytes read from RTC

rtc: ds1390: fix number of bytes read from RTC

The spi_write_then_read() reads 8 bytes starting from
DS1390_REG_SECONDS (== 0x01), so the last byte read would already
be part of the alarm (Tenths and Hundredths of Seconds) feature.

However 7 bytes are engouh -- seconds (0x01), minutes (0x02), hours (0x03),
day (0x04), date (0x05), month/century (0x06) and year (0x07).

Signed-off-by: Andreas Gabriel-Platschek <andi.platschek@gmail.com>
Link: https://patch.msgid.link/20260209053439.313825-1-andi.platschek@gmail.com
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>