ipmi/watchdog: replace atomic_add() and atomic_sub()

atomic_inc() and atomic_dec() looks better

Signed-off-by: Yejune Deng <yejune.deng@gmail.com>
Message-Id: <1605511807-7135-1-git-send-email-yejune.deng@gmail.com>
Signed-off-by: Corey Minyard <cminyard@mvista.com>