ASoC: stm32: spdifrx: fix dma channel release in stm32_spdifrx_remove

In the Linux kernel, the following vulnerability has been resolved:

ASoC: stm32: spdifrx: fix dma channel release in stm32_spdifrx_remove

In case of error when requesting ctrl_chan DMA channel, ctrl_chan is not

null. So the release of the dma channel leads to the following issue:

[ 4.879000] st,stm32-spdifrx 500d0000.audio-controller:

dma_request_slave_channel error -19

[ 4.888975] Unable to handle kernel NULL pointer dereference

at virtual address 000000000000003d

[...]

[ 5.096577] Call trace:

[ 5.099099] dma_release_channel+0x24/0x100

[ 5.103235] stm32_spdifrx_remove+0x24/0x60 [snd_soc_stm32_spdifrx]

[ 5.109494] stm32_spdifrx_probe+0x320/0x4c4 [snd_soc_stm32_spdifrx]

To avoid this issue, release channel only if the pointer is valid.