ARM: riscpc: fix DMA

[ Upstream commit ffd9a1ba ]

DMA got broken a while back in two different ways:
1) a change in the behaviour of disable_irq() to wait for the interrupt
   to finish executing causes us to deadlock at the end of DMA.
2) a change to avoid modifying the scatterlist left the first transfer
   uninitialised.

DMA is only used with expansion cards, so has gone unnoticed.

Fixes: fa4e9989

 ("[ARM] dma: RiscPC: don't modify DMA SG entries")
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: Sasha Levin <sashal@kernel.org>