dell-wmi: Update code for processing WMI events

The WMI buffer can contain multiple events. First value in buffer is
length of event followed by data of specified length. After that is next
length and next data.  When length is zero then there is no more events
in bufffer.

This patch adds support for processing all events in buffer (not only
first) and parse more event types (not only hotkey events). Because of
variable length of events sometimes BIOS fills more hotkeys (or other
values) into single WMI event. In this case this patch also processes
these multiple hotkeys (and not only first one).

Some event types are just ignored because kernel is not interested in
them (e.g. NIC Link status, battery unplug, ...).

This patch is based on DSDT table from Dell Latitude E6440. Code should
be backward compatible so will process other events of old types same as
before this patch.

This patch also fixes a problem with unknown WMI event messages being
written to the log. Now all known events are parsed and those which are
not interesting to the kernel are dropped without an unknown WMI event
message.

Signed-off-by: Pali Rohár <pali.rohar@gmail.com>
Tested-by: Pali Rohár <pali.rohar@gmail.com>
Signed-off-by: Darren Hart <dvhart@linux.intel.com>