spi: spi-fsl-dspi: Increase target mode DMA buffer size

spi: spi-fsl-dspi: Increase target mode DMA buffer size

When the device is configured as a target, the host won't stop sending
data while we're draining the buffer which leads to FIFO underflows
and corruption.

Increase the DMA buffer size to the maximum words that edma can
transfer once to reduce the chance of this happening.

In host mode, the driver is able to split up a transfer into smaller
chunks so we don't need to increase the size. While in target mode, the
length of the transfer is determined by the remote host and can be
larger than whatever default buffer size we pick. Keeping the buffer
small in host mode avoids wasting memory, but allocating the largest
possible in target mode gives the lowest possible chance of dropping any
data from the host.

While we could allocate per-transfer using the exact size of the
transfer, 128K is quite a large allocation and there is a chance it
could fail due to memory fragmentation unless it's allocated once at
init time.

Signed-off-by: Larisa Grigore <larisa.grigore@nxp.com>
Signed-off-by: James Clark <james.clark@linaro.org>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Message-ID: <20250902-james-nxp-spi-dma-v6-6-f7aa2c5e56e2@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>