rpmsg: omaprpc: Fix omaprpc_release hang

omaprpc_release can hang due to an uninterruptible
wait for an ack from the remote core.

Change wait to be interruptible.

Change-Id: Iaf449119b5525db131c536434c84ea296f6e8a95
Signed-off-by: Alberto Aguirre <a-aguirre@ti.com>