rtc: r9701: stop setting RWKCNT

tm_wday is never checked for validity and it is not read back in
r9701_get_datetime. Avoid setting it to stop tripping static checkers:

        drivers/rtc/rtc-r9701.c:109 r9701_set_datetime()
        error: undefined (user controlled) shift '1 << dt->tm_wday'

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20201015191135.471249-4-alexandre.belloni@bootlin.com