Fix the following warning:

commands/help.c: In function ‘list_group’:
commands/help.c:57:4: warning: format not a string literal and
no format arguments [-Wformat-security]
    printf(cmdtp->name);
    ^

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>