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

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.

Fixes: 794df9448edb ("ASoC: stm32: spdifrx: manage rebind issue")
Signed-off-by: Amelie Delaunay <amelie.delaunay@foss.st.com>
Signed-off-by: Olivier Moysan <olivier.moysan@foss.st.com>
Link: https://patch.msgid.link/20241105140242.527279-1-olivier.moysan@foss.st.com
Signed-off-by: Mark Brown <broonie@kernel.org>