Issue in ACPI parsing code

[Tomasz G. <to...@wp...>]

I've found a problem in acpi.cc that caused not parsing acpi tables
properly on my pc. The problem is that there is an incomplete check for
usability of xsdt table. According to some documentation on the net
value 0 in revision entry in rsdp means that this is acpi 1.0 and xsdt
table is not available but this was not checked.

Attached patch fixed this issue for me.

-- 
Tomasz Gajewski