tty: serial: tegra: Optimize DMA buffer synchronization

Synchronize only the dirty part of DMA buffer in order to avoid
unnecessary overhead of syncing of the clean part, which is the case
of every serial DMA transfer in practice.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Link: https://lore.kernel.org/r/20200112180919.5194-3-digetx@gmail.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>