ACPICA: Cleanup byte/word/dword extraction macros, fix possible warnings

Removed unnecessary masking. For the 64-bit macros, removed
the structure overlay. Fixes aliasing warnings seen with gcc 4+
compilers.

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>