dmaengine: dmatest: timeout value of -1 should specify infinite wait

The dmatest module parameter 'timeout' is documented as accepting a
-1 to mean "infinite timeout". However, an infinite timeout is not
advised, nor possible since the module parameter is an unsigned int,
which won't accept a negative value. Change the parameter
comment to reflect current behavior, which allows values from 0 up to
4294967295 (0xFFFFFFFF).

Signed-off-by: Gary R Hook <gary.hook@amd.com>
Signed-off-by: Vinod Koul <vkoul@kernel.org>