i8257: fix Terminal Count status

When a DMA transfer is done (ie all bytes have been transfered), the corresponding
Terminal Count bit must be set in the status register.
This bit is already cleared in i8257_read_cont and i8257_write_cont when required.

This fixes (at least) floppy transfer in IBM 40p firmware, which checks in DMA
controller if everything went fine.

Signed-off-by: Hervé Poussineau <hpoussin@reactos.org>
Message-Id: <1456404332-31556-1-git-send-email-hpoussin@reactos.org>
Reviewed-by: John Snow <jsnow@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
