serial: sirf: don't submit dma desc after timeout irqs occur

In rx timeout ISR and tasklet, we don't issue new dma desc as rx_done ISR
will do that.

Signed-off-by: Qipan Li <Qipan.Li@csr.com>
Signed-off-by: Barry Song <Baohua.Song@csr.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>