rpmsg: Guard against null endpoint ops in destroy

In RPMSG GLINK, the chrdev device will allocate an ept as part of the
rpdev creation. This device will not register endpoint ops even though
it has an ept. Protect against the case where the device is
being destroyed.

Change-Id: If712d8a0a51404e97e246cc2c5085f6b0f6cf8f3
Signed-off-by: Chris Lew <clew@codeaurora.org>