ipmi_si_intf: Fix race in timer shutdown handling

smi_mod_timer() enables the timer before setting timer_running. This
means the timer can be running when we get to stop_timer_and_thread()
without timer_running having been set, resulting in del_timer_sync()
not being called and the timer being left to cause havoc during
shutdown.

Instead just call del_timer_sync() unconditionally

Signed-off-by: Jes Sorensen <jsorensen@fb.com>
Message-Id: <20190828203625.32093-2-Jes.Sorensen@gmail.com>
Signed-off-by: Corey Minyard <cminyard@mvista.com>