diag: Read and drop incoming data when channels are not opened

Diag data channel which is not yet opened is not reading
any data from socket when data is ready, which is resulting
to memory allocation failure on ipc router side after multiple
SSR due to pending qrtr control packets to be read by diag.
The patch handles this case by reading and dropping the data
when it is available.

Change-Id: I4c348c5489923f51caf366d4d08696bff775ae08
Signed-off-by: Hardik Arya <harya@codeaurora.org>