diag: Continue processing of remaining pkt when read len is zero

Currently if read data is len is zero we are failing and stop
reading other data packets. With the patch we continue read
remaining data packets in this case.

Change-Id: I89cd1557bc4b7a46c68a90d2894fd4aaaf881ef5
Signed-off-by: Hardik Arya <harya@codeaurora.org>