cxgb4: Remove redundant NULL check

Fix the following coccicheck warnings:

./drivers/net/ethernet/chelsio/cxgb4/cudbg_lib.c:3540:2-8: WARNING: NULL
check before some freeing functions is not needed.

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Signed-off-by: David S. Miller <davem@davemloft.net>