cpuidle: menu: use DIV_ROUND_CLOSEST_ULL()

Now that the kernel provides DIV_ROUND_CLOSEST_ULL(), drop the internal
implementation and use the kernel one.

Signed-off-by: Javi Merino <javi.merino@arm.com>
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Stephen Hemminger <shemminger@linux-foundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>