bna: fix memory leak during RX path cleanup

The memory leak was caused by unintentional assignment of the Rx path
destroy callback function pointer to NULL just after correct
initialization.

Signed-off-by: Debashis Dutt <ddutt@brocade.com>
Signed-off-by: Rasesh Mody <rmody@brocade.com>
Signed-off-by: David S. Miller <davem@davemloft.net>