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>