soc: qcom: rpmh: Correct rpm_msg pointer offset and add list_del

rpm_msgs address is calculated from memory allocated during write_batch.
Correct the offset from where rpm_msgs actually starts and assign req
pointer to point to correct address.

Also add list_del before freeing memory in invalidate_batch.

Change-Id: Idca43fe73bd238e90911158b08b3889c001e1123
Signed-off-by: Maulik Shah <mkshah@codeaurora.org>