hwmon:ibm: Change ipmi_user_t to struct ipmi_user *

Signed-off-by: Corey Minyard <cminyard@mvista.com>
Cc: Darrick J. Wong <darrick.wong@oracle.com>
Acked-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>