spi: qcom-qspi: Correct max DMA length to avoid 64K boundary failure

spi: qcom-qspi: Correct max DMA length to avoid 64K boundary failure

The maximum size for a DMA data descriptor is 64KB-1 because the size
field in HW is 16 bits wide. For this reason, transfers fail at 64KB
and beyond.

Lower max_dma_len to 60KB so larger transfers are split into multiple
DMA blocks and do not hit the failing 64KB boundary. 60KB is chosen as
a safe round number below the 64KB-1 hardware limit while satisfying
alignment requirements.

Tested on x1e80100 (Hamoa) with SPI-NOR flash (/dev/mtd0):

Without patch:
  dd if=/dev/mtd0 of=/tmp/spi_dump.bin bs=32768 count=2  # works
  dd if=/dev/mtd0 of=/tmp/spi_dump.bin bs=65536 count=1  # fails

With patch:
  dd if=/dev/mtd0 of=/tmp/spi_dump.bin bs=65536 count=1  # works

Fixes: b5762d95607e ("spi: spi-qcom-qspi: Add DMA mode support")
Cc: stable@vger.kernel.org
Signed-off-by: Vijaya Krishna Nivarthi <vijaya.nivarthi@oss.qualcomm.com>
Link: https://patch.msgid.link/20260722092358.459943-1-vnivarth@qti.qualcomm.com
Signed-off-by: Mark Brown <broonie@kernel.org>