PM: sleep: Use ktime_us_delta() in initcall_debug_report()

Use ktime_us_delta() to make the debug log more precise instead of
shifting the return value of ktime_to_ns() applied to a ktime_sub()
result by 10 bit positions to the right.

Signed-off-by: Mark-PK Tsai <mark-pk.tsai@mediatek.com>
[ rjw: Changelog rewrite, subject edits ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>