ACPICA: FADT: Fix extraneous length mismatch warning

Incorrect register length mismatch between the 32 and 64 bit
registers in some cases. Code was was checking the wrong pointer
for non-zero, should be looking at the address within the GAS
structure.

Signed-off-by: Bob Moore <robert.moore@intel.com>
Signed-off-by: Lin Ming <ming.m.lin@intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>