hsu: add a periodic timer to check dma rx channel

A general problem for uart rx dma channel is you never know when
and how much data will be received, so usually preset it a DMA
descriptor with a big size, and rely on DMA RX timeout IRQ to
know there is some data in rx channel.

For a RX data size of multiple of MOTSR, there will be no timeout
IRQ issued, thus OS will never be notified about that.

This is a work around for that, current timer frequency is 5 times
per second, it should vary according to the baud rate

When future silicon version fix the problem, this workaround need
be removed

Signed-off-by: Feng Tang <feng.tang@intel.com>
Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>