ipmi: avoid atomic_inc in exit function

This causes a link failure on ARM in certain configurations,
when we reference each atomic operation from .alt.smp.init in
order to patch out atomics on non-SMP systems:

`.exit.text' referenced in section `.alt.smp.init' of drivers/char/ipmi/ipmi_msghandler.o: defined in discarded section `.exit.text' of drivers/char/ipmi/ipmi_msghandler.o

In this case, we can trivially replace the atomic_inc() with
an atomic_set() that has the same effect and does not require
a fixup.
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Message-Id: <20190415155509.3565087-1-arnd@arndb.de>
Signed-off-by: Corey Minyard <cminyard@mvista.com>

drivers/char/ipmi/ipmi_msghandler.c

@@ -5179,7 +5179,7 @@ static void __exit cleanup_ipmi(void)
 		 * avoids problems with race conditions removing the timer
 		 * here.
 		 */
-		atomic_inc(&stop_operation);
+		atomic_set(&stop_operation, 1);
 		del_timer_sync(&ipmi_timer);
 
 		initialized = false;