dmaengine: dmatest: correct thread_count while using multiple thread per channel

It seems that thread_count is not properly calculated in dmatest.
In fact the thread count number that is returned from dmatest_add_threads() is
not correctly added to the thread_count and thus not properly printed.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: Haavard Skinnemoen <haavard.skinnemoen@atmel.com>
Acked-by: Maciej Sosnowski <maciej.sosnowski@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>