i8k: Add support for Dell Latitude E6540

Add support for the Dell Latitude E6540 which needs a different fan speed multiplier.

Signed-off-by: Steven Honeyman <stevenhoneyman@gmail.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>