serial: sirf: correct condition for fetching dma buffer into tty

In rx dma-callback it calls tasklet_schedule, if the tasklet
be scheduled after all the dma-callback in the rx dma channel,
current check condition in the tasklet will not do fetch dma
buffer into tty because tx_issued is equal with tx_completed,
so as timeout tasklet does.

so we check whether we should fetch the whole dma buffer into
tty according to the status of transactions in rx dma channel.

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>