x86: acpi: limit search on where EBDA can be

This limits the search for Extended BIOS Data Area (EBDA) to
0x80000 to 0x100000 as this is usually the area for it.
If 0000:040e has an address not pointing to this area, it is
probably an invalid address, and should not be de-referenced
to avoid segfault.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>