kernel: timeout: fix issue with z_timeout_expires

- issue found with Ztest case of test_thread_timeout_remaining_expires
  on Intel ISH platform when adjust CONFIG_SYS_CLOCK_TICKS_PER_SEC
  to 10k.
- timeout_rem() return exact remaining ticks which is calibrated by
  decrease elapsed(), while z_timeout_expires try to get expire ticks
  to be timeout using current tick as base, so need get exact current
  ticks by plus elasped().

Signed-off-by: Qipeng Zha <qipeng.zha@intel.com>