i2c: tegra: Fix failure during probe deferral cleanup

If the driver fails to obtain a DMA channel, it will initiate cleanup
and try to release the DMA channel that couldn't be retrieved. This will
cause a crash because the cleanup will try to dereference an ERR_PTR()-
encoded error code.

However, there's nothing to clean up at this point yet, so we can avoid
this by simply resetting the DMA channel to NULL instead of storing the
error code.

Fixes: fcc8a89a

 ("i2c: tegra: Share same DMA channel for RX and TX")
Signed-off-by: Thierry Reding <treding@nvidia.com>
Tested-by: Akhil R <akhilrajeev@nvidia.com>
Reviewed-by: Andi Shyti <andi.shyti@kernel.org>
Signed-off-by: Wolfram Sang <wsa@kernel.org>