diag: Reset rsp_buf_busy flag when switching to USB mode

When a client switches from callback mode to USB mode explicitly,
there is a possibility that the last response is not drained to the
user space. Reset the rsp_buf_busy flag in such scenarios so that
we don't hold on to the response buffer forever.

Change-Id: I0be80174e8193174eb53a768c5ed3a4e322d50ec
Signed-off-by: Ravi Aravamudhan <aravamud@codeaurora.org>