i8k: Add support for configurable maximum fan speed value

Newer Dell systems provide more granular fan speed selection.
Add support for it.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Cc: Andreas Mohr <andi@lisas.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>