qla2xxx: Schedule board_disable only once

There are various callers of qla2x00_check_reg{32,16}_for_disconnect
that may schedule board removal on PCI-disconnect. Test-and-set a
dedicated flag before scheduling board_disable so it is invoked only
once.

Signed-off-by: Joe Lawrence <joe.lawrence@stratus.com>
Acked-by: Chad Dupuis <chad.dupuis@qlogic.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>