Enable Dell All-In-One volume up/down keys

Enable Dell All-In-One volume up/down keys

Enable volume up and down hotkeys on WMI events
GUID 284A0E6B-380E-472A-921F-E52786257FB4 and
GUID 02314822-307C-4F66-bf0E-48AEAEB26CC8.

Also works around a firmware bug where the _WED method
should return an integer containing the key code and in fact
the method returns the key code in element zero of a buffer.

BugLink: http://bugs.launchpad.net/bugs/701530
BugLink: http://bugs.launchpad.net/bugs/676997
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Matthew Garrett <mjg@redhat.com>