x86: Fix vrtc_get_time/set_mmss to use new timespec interface

The patch "x86: Increase precision of x86_platform.get/set_wallclock"
changed the x86 platform set_wallclock/get_wallclock interfaces to
use nsec granular timespecs instead of a second granular interface.

However, that patch missed converting the vrtc code, so this patch
converts those functions to use timespecs.

Many thanks to the kbuild test robot for finding this!

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: John Stultz <john.stultz@linaro.org>