rpmsg: char: Avoid double destroy of default endpoint

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

rpmsg: char: Avoid double destroy of default endpoint

The rpmsg_dev_remove() in rpmsg_core is the place for releasing this default endpoint.

So need to avoid destroying the default endpoint in rpmsg_chrdev_eptdev_destroy(), this should be the same as rpmsg_eptdev_release(). Otherwise there will be double destroy issue that ept->refcount report warning:

refcount_t: underflow; use-after-free.

Call trace: refcount_warn_saturate+0xf8/0x150 virtio_rpmsg_destroy_ept+0xd4/0xec rpmsg_dev_remove+0x60/0x70

The issue can be reproduced by stopping remoteproc before closing the /dev/rpmsgX.