MdePkg/UefiDevicePathLib: Refine the DevPathFromTextiSCSI protocol parsing

MdePkg/UefiDevicePathLib: Refine the DevPathFromTextiSCSI protocol parsing

For current iSCSI protocol parsing, UINT16 truncation may be happened. Since
the Spec already have declaimed that 0 is TCP Protocol and 1+ is reserved, the
parsing can be refined as below:

  if (StrCmp (ProtocolStr, L"TCP") == 0) {
    ISCSIDevPath->NetworkProtocol = 0;
  } else {
    //
    // Undefined and reserved.
    //
    ISCSIDevPath->NetworkProtocol = 1;
  }

Cc: Warner Losh <imp@bsdimp.com>
Cc: Ye Ting <ting.ye@intel.com>
Cc: Fu Siyuan <siyuan.fu@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Wu Jiaxin <jiaxin.wu@intel.com>
Reviewed-by: Warner Losh <imp@bsdimp.com>