qla2xxx: Do not crash system for sp ref count zero

Aovid crashing the system in the scenario where firmware
just completes the command and it can not find the command
during abort mailbox processing. This scenario can lead to
sp reference counter being zero. Instead of crashing the
system, use WARN_ON to print warning in log file.

Signed-off-by: Hiral Patel <hiral.patel@qlogic.com>
Signed-off-by: Himanshu Madhani <himanshu.madhani@qlogic.com>
Signed-off-by: James Bottomley <JBottomley@Odin.com>