Fix dmaengine_submit() return type

desc->tx_submit's return type is dma_cookie_t, not int.  Therefore,
dmaengine_submit() should match this return type as it's just
wrapping this detail.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>