dmaengine: fix NULL pointer in channel unregistration function
In the Linux kernel, the following vulnerability has been resolved:

dmaengine: fix NULL pointer in channel unregistration function

__dma_async_device_channel_register() can fail. In case of failure,
chan->local is freed (with free_percpu()), and chan->local is nullified.
When dma_async_device_unregister() is called (because of managed API or
intentionally by DMA controller driver), channels are unconditionally
unregistered, leading to this NULL pointer:
[    1.318693] Unable to handle kernel NULL pointer dereference at virtual address 00000000000000d0
[...]
[    1.484499] Call trace:
[    1.486930]  device_del+0x40/0x394
[    1.490314]  device_unregister+0x20/0x7c
[    1.494220]  __dma_async_device_channel_unregister+0x68/0xc0

Look at dma_async_device_register() function error path, channel device
unregistration is done only if chan->local is not NULL.

Then add the same condition at the beginning of
__dma_async_device_channel_unregister() function, to avoid NULL pointer
issue whatever the API used to reach this function.