char: ipmi: replace snprintf in show functions with sysfs_emit

coccicheck complains about the use of snprintf() in sysfs show
functions:
WARNING  use scnprintf or sprintf

Use sysfs_emit instead of scnprintf, snprintf or sprintf makes more
sense.

Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: Ye Guojin <ye.guojin@zte.com.cn>
Message-Id: <20211021110608.1060260-1-ye.guojin@zte.com.cn>
Signed-off-by: Corey Minyard <cminyard@mvista.com>