drivers/net/ethernet/chelsio/cxgb4/cxgb4_main.c: remove unnecessary null test...

drivers/net/ethernet/chelsio/cxgb4/cxgb4_main.c: remove unnecessary null test before debugfs_remove_recursive

Fix checkpatch warning:
"WARNING: debugfs_remove_recursive(NULL) is safe this check is probably not required"

Cc: Hariprasad S <hariprasad@chelsio.com>
Cc: netdev@vger.kernel.org
Signed-off-by: Fabian Frederick <fabf@skynet.be>
Signed-off-by: David S. Miller <davem@davemloft.net>