diag: Do not reset SMD buffers if USB write fails

Diag driver resets the SMD buffers when USB write fails which can
result in data from SMD being dropped on the Apps side. Do not
reset the SMD buffers if the write fails so that the peripherals
can restrict sending more data.

Change-Id: I81a87bf828102bd33342ba31836fee9808024352
Signed-off-by: Ravi Aravamudhan <aravamud@codeaurora.org>