rpmsg: glink: Fix channel memory leak

If we stop and start the dsp while channel is open then there is a leak
in the driver as the refcount is not accounted for the open.

This patch checks if the channel is open while running cleanup code
and does an extra kref_put to account for open which would ensure
that channel does not leak.

Originally detected by kmemleak:
  backtrace:
    [<ffffff80088b74d8>] kmemleak_alloc+0x50/0x84
    [<ffffff80081ddbc8>] kmem_cache_alloc_trace+0xd4/0x178
    [<ffffff80086b8bd0>] qcom_glink_alloc_channel+0x34/0x148
    [<ffffff80086b8038>] qcom_glink_work+0x3b0/0x664
    [<ffffff80080c3da8>] process_one_work+0x160/0x2f8
    [<ffffff80080c4198>] worker_thread+0x1e8/0x2d4
    [<ffffff80080c8b24>] kthread+0x128/0x138
    [<ffffff80080845b4>] ret_from_fork+0x10/0x18
    [<ffffffffffffffff>] 0xffffffffffffffff
unreferenced object 0xffffffc02cf5ed80 (size 128):

Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>