spi: ti-qspi: fix use-after-free after DMA setup failure

spi: ti-qspi: fix use-after-free after DMA setup failure

The driver falls back to PIO mode if DMA setup fails during probe.

Make sure to clear the DMA channel pointer also if buffer allocation
fails to avoid passing a pointer to the released channel to the DMA
engine (or trying to free the channel a second time on late probe errors
or driver unbind).

This issue was flagged by Sashiko when reviewing a devres allocation
conversion patch.

Fixes: c687c46e9e45 ("spi: spi-ti-qspi: Use bounce buffer if read buffer is not DMA'ble")
Link: https://sashiko.dev/#/patchset/20260505072909.618363-1-johan%40kernel.org?part=17
Cc: stable@vger.kernel.org	# 4.12
Cc: Vignesh R <vigneshr@ti.com>
Signed-off-by: Johan Hovold <johan@kernel.org>
Link: https://patch.msgid.link/20260512074809.915084-1-johan@kernel.org
Signed-off-by: Mark Brown <broonie@kernel.org>