memory: ti-aemif: fix a potential NULL-pointer dereference

Platform data pointer may be NULL. We check it everywhere but in one
place. Fix it.

Fixes: 8af70cd2 ("memory: aemif: add support for board files")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Cc: stable@vger.kernel.org
Signed-off-by: Olof Johansson <olof@lixom.net>