mmc: tmio_mmc_dma: fix PIO fallback on SDHI

mmc: tmio_mmc_dma: fix PIO fallback on SDHI

commit f936f9b67b7f8c2eae01dd303a0e90bd777c4679 upstream.

I'm testing SH-Mobile SDHI driver in DMA mode with  a new DMA controller  using
'bonnie++' and getting DMA error after which the tmio_mmc_dma.c code falls back
to PIO but all commands time out after that.  It turned out that the fallback
code calls tmio_mmc_enable_dma() with RX/TX channels already freed and pointers
to them cleared, so that the function bails out early instead  of clearing the
DMA bit in the CTL_DMA_ENABLE register. The regression was introduced by commit
162f43e31c5a376ec16336e5d0ac973373d54c89 (mmc: tmio: fix a deadlock).
Moving tmio_mmc_enable_dma() calls to the top of the PIO fallback code in
tmio_mmc_start_dma_{rx|tx}() helps.

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Acked-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Chris Ball <cjb@laptop.org>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>