diag: Handle drops for diag over rpmsg

Handle log drops for modem to apps diag communication over rpmsg.
Changes were made to clear the rpmsg buffers after they are marked
free and delete packets from the rx list only after they are processed.

Change-Id: Iee899640a20f9be0dc8d12428504ad986508ebc2
Signed-off-by: Vivek Golani <vgolani@codeaurora.org>