OvmfPkg/IoMmuDxe: propagate errors from AmdSevInstallIoMmuProtocol()

If we cannot install the IOMMU protocol for whatever reason, exit the
driver with an error. The same is already done for the IOMMU Absent
protocol.

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Brijesh Singh <brijesh.singh@amd.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Tested-by: Brijesh Singh <brijesh.singh@amd.com>
Reviewed-by: Brijesh Singh <brijesh.singh@amd.com>

OvmfPkg/IoMmuDxe/AmdSevIoMmu.c

@@ -499,7 +499,7 @@ EDKII_IOMMU_PROTOCOL  mAmdSev = {
   Initialize Iommu Protocol.
 
 **/
-VOID
+EFI_STATUS
 EFIAPI
 AmdSevInstallIoMmuProtocol (
   VOID
@@ -514,5 +514,5 @@ AmdSevInstallIoMmuProtocol (
                   &gEdkiiIoMmuProtocolGuid, &mAmdSev,
                   NULL
                   );
-  ASSERT_EFI_ERROR (Status);
+  return Status;
 }