intel_idle: Clean up irtl_2_usec()

Move the irtl_ns_units[] definition into irtl_2_usec() which is the
only user of it, use div_u64() for the division in there (as the
divisor is small enough) and use the NSEC_PER_USEC symbol for the
divisor.  Also convert the irtl_2_usec() comment to a proper
kerneldo one.

No intentional functional impact.
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/idle/intel_idle.c

@@ -1292,16 +1292,17 @@ static void __init ivt_idle_state_table_update(void)
 	/* else, 1 and 2 socket systems use default ivt_cstates */
 }
 
-/*
- * Translate IRTL (Interrupt Response Time Limit) MSR to usec
+/**
+ * irtl_2_usec - IRTL to microseconds conversion.
+ * @irtl: IRTL MSR value.
+ *
+ * Translate the IRTL (Interrupt Response Time Limit) MSR value to microseconds.
  */
-
-static const unsigned int irtl_ns_units[] __initconst = {
-	1, 32, 1024, 32768, 1048576, 33554432, 0, 0
-};
-
 static unsigned long long __init irtl_2_usec(unsigned long long irtl)
 {
+	static const unsigned int irtl_ns_units[] __initconst = {
+		1, 32, 1024, 32768, 1048576, 33554432, 0, 0
+	};
 	unsigned long long ns;
 
 	if (!irtl)
@@ -1309,8 +1310,9 @@ static unsigned long long __init irtl_2_usec(unsigned long long irtl)
 
 	ns = irtl_ns_units[(irtl >> 10) & 0x7];
 
-	return div64_u64((irtl & 0x3FF) * ns, 1000);
+	return div_u64((irtl & 0x3FF) * ns, NSEC_PER_USEC);
 }
+
 /*
  * bxt_idle_state_table_update(void)
  *