ipmi_si: Potential array underflow in hotmod_handler()

The "ival" variable needs to signed so that we don't read before the
start of the str[] array.  This would only happen the user passed in a
module parameter that was just comprised of space characters.

Fixes: e80444ae4fc3 ("ipmi_si: Switch hotmod to use a platform device")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Message-Id: <20190222195530.GA306@kadam>
Signed-off-by: Corey Minyard <cminyard@mvista.com>