rpmsg: glink: spi: unregister rpdev during ept destroy

Rpmsg device unregister is not happening if channel close is triggered
from local side and causing re-registration of device failures.

Unregister rpmsg device for local close in endpoint destroy path.

Change-Id: Ic0c66346577cf63546c13b341c6313a8d478d8e5
Signed-off-by: Chris Lew <clew@codeaurora.org>