qrtr: Add support to read qrtr packet size

Some transports need to know qrtr packet to read complete packet
from underlying transport.

Add API support to get the incoming packet size form qrtr header.

Change-Id: I4e4e4ab2e4c1fe0e1e1261af85a8b8618ce65bb3
Signed-off-by: Arun Kumar Neelakantam <aneela@codeaurora.org>