ipmi: Fix pr_fmt to avoid compilation issues

[ Upstream commit 2ebaf18a0b7fb764bba6c806af99fe868cee93de ]

The was it was wouldn't work in some situations, simplify it.  What was
there was unnecessary complexity.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Corey Minyard <cminyard@mvista.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>