ide: Reset current_addr after stopping DMA

Whenever SSBM is reset in the command register all state information is lost.
Restarting DMA means that current_addr must be reset to the base address of the
PRD table. The OS is not required to change the base address register before
starting a DMA operation, it can reuse the value it wrote for an earlier
request.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>