atp870u: Request IRQ later, remove weird locking

Allocate IRQ later during probe to avoid code duplication and also
remove the need for weird locking in _probe.
(It was probably there to prevent race with the IRQ handler?)

Signed-off-by: Ondrej Zary <linux@rainbow-software.org>
Reviewed-by: Hannes Reinicke <hare@suse.de>
Acked-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>