scsi: lpfc: Maintain atomic consistency of queue_claimed flag

A previous change introduced the atomic use of queue_claimed flag for eq's
and cq's.  The code works fine, but the clearing of the queue_claimed flag
is not atomic.

Change queue_claimed = 0 into xchg(&queue_claimed, 0) to be consistent for
change under atomicity.

Link: https://lore.kernel.org/r/20200501214310.91713-3-jsmart2021@gmail.com
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Dick Kennedy <dick.kennedy@broadcom.com>
Signed-off-by: James Smart <jsmart2021@gmail.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>