tty/serial: at91: fix rx ring buffer management

tty/serial: at91: fix rx ring buffer management

This patch swaps the use "tail" and "head" to fit the semantic of the linux
circular buffer documentation:
- head: the point at which the producer (the DMA controller) inserts items.
- tail: the point at which the consumer (the serial framework) finds the next
        item.

Besides the former code of the rx ring buffer didn't manage the case where
head < tail, which might lead to loss of data. To fix this bug the data are now
sent from the DMA buffer to the serial framework in two steps:
1 - First, we test if head < tail. If so, we copy the data from tail to the end
    of the DMA buffer then reset tail to zero.
2 - Finally, we copy data from tail to head then set tail to head.

In addition, since tty_insert_flip_string() may now be called twice,
atmel_flip_buffer_rx_dma() becomes less efficient than moving the calls
dma_sync_sg_for_cpu(), dma_sync_sg_for_device(), tty_insert_flip_string() and
tty_flip_buffer_push() directly into atmel_rx_from_dma().

Signed-off-by: Cyrille Pitchen <cyrille.pitchen@atmel.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>