skge: Adjust a null pointer check in skge_down()

The script "checkpatch.pl" pointed information out like the following.

Comparison to NULL could be written "!skge->mem".

Thus fix the affected source code place.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Signed-off-by: David S. Miller <davem@davemloft.net>