diag: Stop reading from peripherals during close

While switching logging modes there is a possibility that the previous
read queued to the transport layer may still be pending. This read
should be cancelled before closing the channel. Otherwise, while
opening the channel again, we may queue the same buffer twice for
reading.

Change-Id: Ic5fb7713876e245bcc3260a9e67e35e611cd93a4
Signed-off-by: Ravi Aravamudhan <aravamud@codeaurora.org>