ATM-nicstar: Refactor a dev_alloc_skb() call in dequeue_rx()

The script "checkpatch.pl" can point out that assignments should usually
not be performed within condition checks.
Thus move an assignment for a local variable to a separate statement
in this function.

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