net: qrtr: mhi_dev: Only post valid data

mhi_dev_read_channel returns the number of bytes read. Do not post a
packet to qrtr if the length returned is 0. This prevents an issue
where qrtr receives duplicates packets from the mhi_dev transport.

Change-Id: Ib1a31665730293baa9af61a9ae61bd8bdf6cab6f
Signed-off-by: Sricharan R <sricharan@codeaurora.org>
Signed-off-by: Chris Lew <clew@codeaurora.org>