rpmsg: glink: Don't send pending rx_done during remove

Attempting to transmit rx_done messages after the GLINK instance is
being torn down will cause use after free and memory leaks. So cancel
the intent_work and free up the pending intents.

With this there are no concurrent accessors of the channel left during
qcom_glink_native_remove() and there is therefor no need to hold the
spinlock during this operation - which would prohibit the use of
cancel_work_sync() in the release function. So remove this.

Fixes: 1d2ea36e

 ("rpmsg: glink: Add rx done command")
Cc: stable@vger.kernel.org
Acked-by: Chris Lew <clew@codeaurora.org>
Tested-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>