diag: Read the DIAG_CMD channel until there is data available

Currently, there are few smd events which are not processed by
diag driver. This leads to the pile of responses committed by
peripherals over the DIAG_CMD channel. This patch continues to
read the DIAG_CMD channel until there is data on that channel.

Change-Id: Ie9c6c84f9f4fd3dc664e46871032def6d894b73c
Signed-off-by: Katish Paran <kparan@codeaurora.org>