rpmsg: glink: Free pending deferred work on remove

By just cancelling the deferred rx worker during GLINK instance teardown
any pending deferred commands are leaked, so free them.

Fixes: b4f8e52b

 ("rpmsg: Introduce Qualcomm RPM glink driver")
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>