ipmi: simplify sysctl registration

register_sysctl_table() is a deprecated compatibility wrapper.
register_sysctl() can do the directory creation for you so just use
that.

Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
Message-Id: <20230302204612.782387-3-mcgrof@kernel.org>
Signed-off-by: Corey Minyard <cminyard@mvista.com>