ACPI / cpuidle: Drop unnecessary calls from acpi_idle_do_entry()

Since the cpuidle core calls stop_critical_timings() and
start_critical_timings() around the execution of ->enter callbacks,
acpi_idle_do_entry() doesn't need to do that (and really shouldn't).

Also using "inline" on it is pointless and the kerneldoc entry does
not reflect the actual situation any more.

Fix all of the above.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>