Description: dmaengine: fix NULL pointer in channel unregistration function
References:
Notes:
 carnil> Introduced in d2fb0a043838 ("dmaengine: break out channel registration").
 carnil> Vulnerable versions: 5.6-rc1.
Bugs:
upstream: released (6.8-rc1) [f5c24d94512f1b288262beda4d3dcb9629222fc7]
6.7-upstream-stable: released (6.7.3) [9263fd2a63487c6d04cbb7b74a48fb12e1e352d0]
6.6-upstream-stable: released (6.6.15) [7f0ccfad2031eddcc510caf4e57f2d4aa2d8a50b]
6.1-upstream-stable: released (6.1.76) [2ab32986a0b9e329eb7f8f04dd57cc127f797c08]
5.10-upstream-stable: released (5.10.210) [9de69732dde4e443c1c7f89acbbed2c45a6a8e17]
4.19-upstream-stable: N/A "Vulnerable code not present"
sid: released (6.6.15-1)
6.1-bookworm-security: released (6.1.76-1)
5.10-bullseye-security: needed
4.19-buster-security: N/A "Vulnerable code not present"