dmaengine: ppc4xx: fix off-by-one build failure

There are two poly_store, but one should have been poly_show.

|adma.c:4382:16: error: conflicting types for 'poly_store'
| static ssize_t poly_store(struct device_driver *dev, const char *buf,
|                ^~~~~~~~~~
|adma.c:4363:16: note: previous definition of 'poly_store' was here
| static ssize_t poly_store(struct device_driver *dev, char *buf)
|                ^~~~~~~~~~

CC: stable@vger.kernel.org
Fixes: 13efe1a0 ("dmaengine: ppc4xx: remove DRIVER_ATTR() usage")
Signed-off-by: Christian Lamparter <chunkeey@gmail.com>
Signed-off-by: Vinod Koul <vkoul@kernel.org>