diag: dci: Handle dci mutex correctly

diag driver uses dci_mutex to protect the global buffer for dci.
While sending a packet to the peripheral, this mutex is not
released on error. This patch release the mutex on error.

Change-Id: Idd84e6533b2c8e748d9761e061d06e188a5a4bac
Signed-off-by: Ravi Aravamudhan <aravamud@codeaurora.org>