dmaengine: altera: Use macros instead of structs to describe the registers

This patch moves from a struct declaration for the DMA controller
registers to macros with offests to the base address. This is mainly
done to remove the sparse warnings, since the function parameter of
ioread32/iowrite32 is "void __iomem *" instead of a pointer to struct
members. With this patch applied, no sparse warning is seen anymore.

Please note that the struct for the descriptors is still kept in place,
as the code largely accesses the struct members as internal variables
before the complete struct is copied into the descriptor FIFO of the
DMA controller.

Additionally this patch also removes two warnings "variable xxx set but
not used" seen when compiling with "W=1". The registers need to be read
to flush the response FIFO, but nothing needs to be done with them. So
the code is correct here and the warning is a false one.

Signed-off-by: Stefan Roese <sr@denx.de>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>