next-cube: always use retval to return rtc read values

next-cube: always use retval to return rtc read values

Instead of shifting out rtc read values from individual rtc registers, change
the logic so that rtc read commands are executed when the last bit of the rtc
command is received and the result stored in retval. This simplifies the rtc
read logic such that the shift out logic can be consolidated for rtc phases
between 8 and 16.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Message-ID: <20241222130012.1013374-22-mark.cave-ayland@ilande.co.uk>
Signed-off-by: Thomas Huth <huth@tuxfamily.org>