sata_dwc_460ex: fix error path

Fixed hsdev memleak on sata_dwc_probe() error.
As dma_dwc_exit() can be called multiple times without sata_dma_regs and
irq_dma changes, it might lead to double free on sequential
dma_dwc_exit() calls.  So, zero these fields after free calls.

Signed-off-by: Vasiliy Kulikov <segoon@openwall.com>
Signed-off-by: Jeff Garzik <jgarzik@pobox.com>