thinkpad-acpi: constrain IBM-era support to IBM boxes

Lenovo is playing around with its ACPI BIOS, and will end up reusing
method names.  Their memory is not nearly as long as thinkpad-acpi's...

Secure most of the old IBM codepaths against running in a non-IBM box.
This would happen on the Lenovo X100e in video_init(), for example.  We
would misdetect it as an ancient model 570 firmware.

Also, refuse to load the driver if we cannot identify the vendor.  No
ACPI ThinkPad in existence lacks this information, AFAIK.

Signed-off-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>