mtd: s3c2410: Fix potential NULL pointer dereference error

'set' is tested for NULL. But subsequently accessed without the check.
Thus making it conditional to avoid NULL pointer dereferencing.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>