firmware/dmi: Include product_sku info to modalias

Some Dell platforms rely on modalias to customize configuration,
the product sku can be more specific for the hardware.

Add product_sku to modalias for better utilization.

Signed-off-by: Kai-Chuan Hsieh <kaichuan.hsieh@canonical.com>
Signed-off-by: Jean Delvare <jdelvare@suse.de>