powercap: idle_inject: use us_to_ktime() where appropriate

Convert values in microseconds to ktime using us_to_ktime() instead of
multiplying them by NSEC_PER_USEC and using ns_to_ktime() for the
conversion.

Signed-off-by: Xichao Zhao <zhao.xichao@vivo.com>
Link: https://patch.msgid.link/20250813075433.464786-1-zhao.xichao@vivo.com


[ rjw: Subject and changelog edits ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>