qdss_bridge: fix use-after-free is on cdev_put

"drvdata->cdev" will be automatically freed when mhi_device
detach, this change use cdev_alloc to allocate cdev to avoid
this issue.

Change-Id: I08d2cd707ebf1768e0dadbdc5ca632f4598104f4
Signed-off-by: Yuanfang Zhang <zhangyuanfang@codeaurora.org>