drivers: rtc: correctly set week day for mc146818

The driver sets the weekday incorrectly when called by the
'date set' command.

Sunday is 1, Saturday is 7 unlike in U-Boot (see data sheet
https://www.nxp.com/docs/en/data-sheet/MC146818.pdf

, table 3).

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>