ipmi: msghandler: Fix a signedness bug

ipmi: msghandler: Fix a signedness bug

The type for the completion codes should be unsigned char instead of
char.  If it is declared as a normal char then the conditions in
__get_device_id() are impossible because the IPMI_DEVICE_IN_FW_UPDATE_ERR
error codes are higher than 127.

    drivers/char/ipmi/ipmi_msghandler.c:2449 __get_device_id()
    warn: impossible condition '(bmc->cc == 209) => ((-128)-127 == 209)'

Fixes: f8910ffa81b0 ("ipmi:msghandler: retry to get device id on an error")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Message-Id: <20200918142756.GB909725@mwanda>
Signed-off-by: Corey Minyard <cminyard@mvista.com>