ACPI: Minor cleanup eliminating redundant PMTIMER_TICKS to NS conversion

acpi_enter_[simple,bm] does
idle timing in ns, convert it to timeval, then to us, then to
pmtimer_ticks and then back to ns.

This patch changes things to
idle timing in ns, convert it to us, and then to pmtimer_ticks.

Just saves an imul along this path, but makes the code cleaner.

Signed-off-by: Venkatesh Pallipadi <venki@google.com>
Signed-off-by: Len Brown <len.brown@intel.com>