rpmsg: glink: Release idr lock before returning on error

The idr_lock should be released in the case that we don't find the given
channel.

Fixes: 44f6df92

 ("rpmsg: glink: Fix idr_lock from mutex to spinlock")
Reported-by: Julia Lawall <julia.lawall@lip6.fr>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>