dmatest: fix max channels handling

The check for reaching max_channels is short circuited by 'continuing'
after successfully adding a channel.

[ Impact: make the 'max_channels' module parameter actually have an effect ]

Cc: <stable@kernel.org>
Reported-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>