ARM: S3C24XX: Fix potential NULL pointer dereference error

chan->end is tested for being NULL. However in the event that it is NULL, the
subsequent assignment statement would lead to NULL pointer dereference.
Thus dereferencing it only when it is not NULL.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>