dell-wmi: mask off upper bytes of event response

In debugging with some future machines that actually contain BIOS level
support for dell-wmi, I've determined that the upper half of the data that
comes back from wmi_get_event_data may sometimes contain extra information
that isn't currently relevant when pulling scan codes out of the data.
This causes dell-wmi to improperly respond to these events.

Signed-off-by: Mario Limonciello <mario_limonciello@dell.com>
Signed-off-by: Matthew Garrett <mjg@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Len Brown <len.brown@intel.com>