dmaengine: mpc512x: Fix hanging DMA device transfer for MPC8308

Since the MPC8308 has no external request lines to initiate DMA transfers,
all transfers must be triggered by software.

Because of this, the current implementation of DMA transfers from and to
devices on MPC8308 SoCs using major and minor loops is faulty: After the
completion of the first major loop, the DMA engine resets the start flag in
the channel's TCD, thus halting the transfer. The driver would have to set
the start bit again to trigger the next iteration of the major loop; on
MPC512x SoCs, this is done via the external request lines, so in this case,
the driver doesn't have to interfer in any way.

This has the effect that on MPC8308s, every DMA transfer to or from a
device hangs after executing the first major loop.

The patch fixes this behavior by using just one major loop for the whole
DMA transfer on MPC8308s.

Signed-off-by: Mario Six <mario.six@gdsys.cc>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>