dmaengine: tegra: make byte counters unsigned int

The buffer byte request length and counter are declared as signed integers
but the values should never be below zero, so make these unsigned integers
instead.

Reviewed-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Signed-off-by: Vinod Koul <vkoul@kernel.org>