soc: qcom: pmic_glink: Fix a race condition in accessing rx_list

Currently, rx_list is updated from pmic_glink_rpmsg_callback()
with the new message received and the workqueue is queued for
pmic_glink_rx_work() to deliver it to the client. However, there
is a possibility that a race condition can occur when this list
is manipulated from pmic_glink_rpmsg_callback() in interrupt
context and pmic_glink_rx_work() from process context. Fix it by
using rx_lock before using list_* in pmic_glink_rx_work().

Change-Id: Ib10671d89d60e4b33f1a9666f4af08bb09d1a53b
Suggested-by: Chris Lew <clew@codeaurora.org>
Suggested-by: David Collins <collinsd@codeaurora.org>
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>