spi: ep93xx: fix error pointer deref after DMA setup failure

spi: ep93xx: fix error pointer deref after DMA setup failure

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

Make sure to the clear the DMA channel pointers on setup failure to
avoid dereferencing an error pointer on later probe errors or driver
unbind.

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

Fixes: e79e7c2df627 ("spi: ep93xx: add DT support for Cirrus EP93xx")
Link: https://sashiko.dev/#/patchset/20260429091333.165363-1-johan%40kernel.org?part=10
Cc: stable@vger.kernel.org	# 6.12
Cc: Nikita Shubin <nikita.shubin@maquefel.me>
Signed-off-by: Johan Hovold <johan@kernel.org>
Acked-by: Nikita Shubin <nikita.shubin@maquefel.me>
Link: https://patch.msgid.link/20260512074849.915143-1-johan@kernel.org
Signed-off-by: Mark Brown <broonie@kernel.org>