msm: rpm-smd-debug: Disable rpm_send_msg by default

rpm_send_msg is only intended for manual debugging only. Disable it by
default. A magic is required to enable this debug functionality.

In addition, keep track of number of messages sent in
rpm_send_msg_usage_count.

Change-Id: I31e094097c3a99fbf380d23cf2b5dca4b5143955
Signed-off-by: Junjie Wu <junjiew@codeaurora.org>