diag: Reset read buffers on interrupts

Diag driver needs to reset the read buffers in case the read thread
is interrupted by a signal. This ensures that the buffer is
available for reading subsequent packets from peripherals. This
patch also changes the wait_event to wait_event_interruptible while
reading data from SMD.

Change-Id: Ic8fc919225e3b7f3466bb7c584776afd636bacc4
Signed-off-by: Ravi Aravamudhan <aravamud@codeaurora.org>