dell-wmi: Check if Dell WMI descriptor structure is valid

After examining existing DSDT ACPI tables of more laptops and looking
into Dell WMI document mentioned in ML dicussion archived at
http://www.spinics.net/lists/platform-driver-x86/msg07220.html

 we will
parse and check WMI descriptor if contains expected data. It is because
WMI descriptor contains interface version number and it is needed to
know in next commit.

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>