ipmi: Use is_visible callback for conditional sysfs entries

Instead of manual calls of device_create_file() and
device_remove_file(), implement the condition in is_visible callback
for the attribute group and put these entries to the group, too.
This simplifies the code and avoids the possible races.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Corey Minyard <cminyard@mvista.com>