[SCSI] qla4xxx: Clear interrupt while unloading driver.

while processing the stop firmware mailbox command on driver unload,
a interrupt is processed which causes kernel panic as the response
queue is not valid.

Fix is to clear the interrupt in free_adapter call just after
disabling the interrupts.

Signed-off-by: Vikas Chaudhary <vikas.chaudhary@qlogic.com>
Reviewed-by: Mike Christie <michaelc@cs.wisc.edu>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>