dmaengine: altera: fix response FIFO emptying

Commit 6084fc2e

 ("dmaengine: altera: Use macros instead of structs
to describe the registers") introduced a minus sign before a register
offset.

This leads to soft-locks of the DMA controller, since reading the last
status byte is required to pop the response from the FIFO. Failing to
do so will lead to a full FIFO, which means that the DMA controller
will stop processing descriptors.

Signed-off-by: Sylvain Lesne <lesne@alse-fr.com>
Reviewed-by: Stefan Roese <sr@denx.de>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>