Input: wistron - do not crash if BIOS does not support interface

offset can never be < 0 because it has type size_t.  The driver
currently oopses on insmod if BIOS does not support the interface,
instead of refusing to load.

Signed-off-by: Miloslav Trmac <mitr@volny.cz>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>