Per RFC 1350, TFTP error packets include 2 byte OpCode and ErrorCode
fields in network byte order. Those need to be swapped to host order to
be interpreted correctly. Without this change, the TftpErrorReceived and
TftpError Mode fields are never set and EFI applications can't inspect
the error received from the TFTP server.
Signed-off-by: Dan Nicholson <dbn@endlessos.org>