Utilities: Cleanup style issue for bit clearing

It's reported in Linux community that change to utmath.c contains a style
problem:
 [.../utmath.c:137]: (style) Same expression on both sides of '^='.
 [.../utmath.c:174]: (style) Same expression on both sides of '^='.
This patch fixes this problem. ACPICA BZ 1422, reported by David Binderman,
fixed by Lv Zheng.

Link: https://bugs.acpica.org/show_bug.cgi?id=1422
Reported-by: David Binderman <dcb314@hotmail.com>
Signed-off-by: Lv Zheng <lv.zheng@intel.com>

source/components/utilities/utmath.c

@@ -260,7 +260,7 @@ AcpiUtShortShiftLeft (
     if ((Count & 63) >= 32)
     {
         OperandOvl.Part.Hi = OperandOvl.Part.Lo;
-        OperandOvl.Part.Lo ^= OperandOvl.Part.Lo;
+        OperandOvl.Part.Lo = 0;
         Count = (Count & 63) - 32;
     }
     ACPI_SHIFT_LEFT_64_BY_32 (OperandOvl.Part.Hi,
@@ -305,7 +305,7 @@ AcpiUtShortShiftRight (
     if ((Count & 63) >= 32)
     {
         OperandOvl.Part.Lo = OperandOvl.Part.Hi;
-        OperandOvl.Part.Hi ^= OperandOvl.Part.Hi;
+        OperandOvl.Part.Hi = 0;
         Count = (Count & 63) - 32;
     }
     ACPI_SHIFT_RIGHT_64_BY_32 (OperandOvl.Part.Hi,