Comment 5 for bug 1143980

Jeff Lane
[ 0.498890] [Firmware Bug]: ACPI: BIOS _OSI(Linux) query ignored

Hence, as per http://www.dell.com/support/drivers/us/en/19/Product/latitude-6430u-ultrabook an update is available for your BIOS (A06). If you update to this following https://help.ubuntu.com/community/BiosUpdate , does it change anything? If it doesn't, could you please both specify what happened, and just provide the output of the following terminal command:
sudo dmidecode -s bios-version && sudo dmidecode -s bios-release-date

Thank you for your understanding.