dmaengine: stm32-dma3: improve residue granularity

Implement own device_tx_status ops to compute the residue with a finer
granularity, up to bytes.
STM32 DMA3 has a bitfield, BNDT, in CxTR1 register which reflects the
number of bytes read from the source.
It also has a bitfield, FIFOL, in CxSR register which reflects the FIFO
level in units of programmed destination data width.
The channel is briefly suspended to get a coherent snapshot of registers.
It is possible to correct the fifo level when packing/unpacking is enabled
with destination increment.

Signed-off-by: Amelie Delaunay <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Vinod Koul <[email protected]>

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