!5195 dmaengine: fix NULL pointer in channel unregistration function

Merge Pull Request from: @ci-robot 
 
PR sync from: Zheng Yejian <zhengyejian1@huawei.com>
https://mailweb.openeuler.org/hyperkitty/list/kernel@openeuler.org/message/O72YUEP2GLKL2NC6L4K3AZTXAWXJMML5/ 
 
https://gitee.com/src-openeuler/kernel/issues/I97NHW 
 
Link:https://gitee.com/openeuler/kernel/pulls/5195

 

Reviewed-by: Xu Kuohai <xukuohai@huawei.com>
Signed-off-by: Jialin Zhang <zhangjialin11@huawei.com>