mc146818rtc: Update the Day of Week in CMOS in the right way.

mc146818rtc: Update the Day of Week in CMOS in the right way.

The range of weekday in CMOS is 01-07, while the Sunday is 1, and
Saturday is 7. The comand date in coreutils defines
  %u   day of week (1..7); 1 is Monday
  %w   day of week (0..6); 0 is Sunday
There are 1 day offset for each week day. So we use "%w" and plus 1
before we update the weekday in CMOS.

Change-Id: I3fab4e95f04924ff0ba10a7012b57da1d3f0d1a5
Signed-off-by: Zheng Bao <zheng.bao@amd.com>
Signed-off-by: zbao <fishbaozi@gmail.com>
Reviewed-on: http://review.coreboot.org/1802
Tested-by: build bot (Jenkins)
Reviewed-by: Anton Kochkov <anton.kochkov@gmail.com>