ipmi: use correct string length

gcc-8 reports

drivers/char/ipmi/ipmi_msghandler.c: In function
'panic_op_write_handler':
./include/linux/string.h:245:9: warning: '__builtin_strncpy' specified
bound 16 equals destination size [-Wstringop-truncation]

drivers/char/ipmi/ipmi_watchdog.c: In function 'set_param_str':
./include/linux/string.h:245:9: warning: '__builtin_strncpy' specified
bound 16 equals destination size [-Wstringop-truncation]

We need one less byte or call strlcpy() to make it a nul-terminated
string.

Signed-off-by: Xiongfeng Wang <xiongfeng.wang@linaro.org>
Signed-off-by: Corey Minyard <cminyard@mvista.com>