dell-wmi: Process only one event on devices with interface version 0

dell-wmi: Process only one event on devices with interface version 0

BIOS/ACPI on devices with WMI interface version 0 does not clear buffer
before filling it. So next time when BIOS/ACPI send WMI event which is
smaller as previous then it contains garbage in buffer from previous event.

BIOS/ACPI on devices with WMI interface version 1 clears buffer and
sometimes send more events in buffer at one call.

Since commit 35b4c98c4086 ("dell-wmi: Update code for processing WMI
events") dell-wmi process all events in buffer (and not just first).

To prevent reading garbage from the buffer we process only the first
event on devices with WMI interface version 0.

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