qlcnic: Flush mailbox command list when mailbox is not available

o Driver was hitting a panic at the time of adapter reset due to invalid command
  access from the list which had been already freed by the queuing thread.
  Flush all the pending commands from the list before proceeding with adapter reset

Signed-off-by: Manish Chopra <manish.chopra@qlogic.com>
Signed-off-by: Sucheta Chakraborty <sucheta.chakraborty@qlogic.com>
Signed-off-by: David S. Miller <davem@davemloft.net>