spi: dw: Use DMA max burst to set the request thresholds

Each channel of DMA controller may have a limited length of burst
transaction (number of IO operations performed at ones in a single
DMA client request). This parameter can be used to setup the most
optimal DMA Tx/Rx data level values. In order to avoid the Tx buffer
overrun we can set the DMA Tx level to be of FIFO depth minus the
maximum burst transactions length. To prevent the Rx buffer underflow
the DMA Rx level should be set to the maximum burst transactions length.
This commit setups the DMA channels and the DW SPI DMA Tx/Rx levels
in accordance with these rules.

Signed-off-by: Serge Semin <[email protected]>
Reviewed-by: Andy Shevchenko <[email protected]>
Cc: Alexey Malahov <[email protected]>
Cc: Thomas Bogendoerfer <[email protected]>
Cc: Arnd Bergmann <[email protected]>
Cc: Feng Tang <[email protected]>
Cc: Rob Herring <[email protected]>
Cc: [email protected]
Cc: [email protected]
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Mark Brown <[email protected]>

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