rtc: rv3028: correct weekday register usage

The datasheet for the rv3028 says the weekday has exact 3 bits
and in chapter 3.4.0 for the "3h–Weekday" register it says:
"""
This register holds the current day of the week. Each value represents
one weekday that is assigned by the user. Values will range from 0 to 6
The weekday counter is simply a 3-bit counter which counts up to 6
and then resets to 0.
"""

So do not code weekday bitwise instead, use the raw values from
0-6.

Tested on "PHYTEC phyBOARD-Pollux i.MX8MP" board.

Signed-off-by: Heiko Schocher <hs@denx.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20210309134719.1494062-1-hs@denx.de