dmaengine: dw-axi-dmac: Allow client-chosen width

[pelwell]

For devices where transfer lengths are not known upfront, there is a
danger when the destination is wider than the source that partial words
can be lost at the end of a transfer. Ideally the controller would be
able to flush the residue, but it can't - it's not even possible to tell
that there is any.

Instead, allow the client driver to avoid the problem by setting a
smaller width.

[pelwell]

@P33M The cause of the recent UART woes was the loss of this code fragment in the DMAC driver:

               /* Prevent partial access units getting lost */
               if (mem_width > reg_width)
                       mem_width = reg_width;

The partial access units in question are the bytes that have been received from the UART but not yet grouped into a word. If the UART driver decides that sufficient time has passed that any residue should be pushed to the tty framework, this partial word is inaccessible. And unlike the transmit case, there is no way of knowing what those bytes were.

Work around the problem by honouring a non-zero memory destination width (unless the DMAC driver knows it must be more restrictive).