MdeModulePkg: Initialize local variable value before they are used

BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=3228
This change is to fix the false compiler error on GCC49 release build.

Cc: Jian J Wang <jian.j.wang@intel.com>
Cc: Hao A Wu <hao.a.wu@intel.com>
Signed-off-by: Liming Gao <gaoliming@byosoft.com.cn>
Reviewed-by: Hao A Wu <hao.a.wu@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>

MdeModulePkg/Bus/Ata/AtaAtapiPassThru/AhciMode.c

@@ -1127,6 +1127,11 @@ AhciDmaTransfer (
     return EFI_INVALID_PARAMETER;
   }
 
+  //
+  // Set Status to suppress incorrect compiler/analyzer warnings
+  //
+  Status = EFI_SUCCESS;
+
   //
   // DMA buffer allocation. Needs to be done only once for both sync and async
   // DMA transfers irrespective of number of retries.