rpmsg: glink: use correct data pointer in packets larger than 8k

Only first 8K data is sending repeatedly in split and sending case.

Update data pointer with correct index while sending split packets.

CRs-Fixed: 2391918
Change-Id: I48925c0249c8849a6df388de1b98aa5b2a524e2e
Signed-off-by: Arun Kumar Neelakantam <aneela@codeaurora.org>