Fix the includes in regulator_fixed.c

Include sys/bus.h directly rather than depend on header pollution.
While here fix the order of sys/systm.h.

Sponsored by:	Innovate UK

sys/dev/extres/regulator/regulator_fixed.c

@@ -29,11 +29,12 @@ __FBSDID("$FreeBSD$");
 
 #include "opt_platform.h"
 #include <sys/param.h>
+#include <sys/systm.h>
+#include <sys/bus.h>
 #include <sys/conf.h>
 #include <sys/gpio.h>
 #include <sys/kernel.h>
 #include <sys/kobj.h>
-#include <sys/systm.h>
 #include <sys/module.h>
 #include <sys/mutex.h>