bna: Fix for handling firmware heartbeat failure

This patch contains a fix for gracefully handling firmware heartbeat
failure instead of forcing panic.

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>