rpmsg: fix name service endpoint leak

The name service endpoint wasn't destroyed, so fix it.

This is achieved by introducing an internal __rpmsg_destroy_ept
function which doesn't assume the given ept is bound to an rpmsg
channel (much like the existing __rpmsg_create_ept).

This is needed because the name service ept belongs to the rpmsg bus,
and is never bound with a specific rpdev.

Reported-by: Omar Ramirez Luna <omar.ramirez@ti.com>
Signed-off-by: Ohad Ben-Cohen <ohad@wizery.com>
Cc: Grant Likely <grant.likely@secretlab.ca>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Mark Grosen <mgrosen@ti.com>
Cc: Suman Anna <s-anna@ti.com>
Cc: Fernando Guzman Lugo <fernando.lugo@ti.com>
Cc: Rob Clark <rob@ti.com>
Cc: Ludovic BARRE <ludovic.barre@stericsson.com>
Cc: Loic PALLARDY <loic.pallardy@stericsson.com>
Cc: Omar Ramirez Luna <omar.ramirez@ti.com>