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: NJulia Lawall <julia.lawall@lip6.fr>
Signed-off-by: NBjorn Andersson <bjorn.andersson@linaro.org>