diag: Fix issue while sending responses to user space clients

Diag driver can miss sending responses to clients in Memory Device
Mode due to a race condition. Check and correct the condition and
send responses in those cases.

Change-Id: Id4b4f51272e97ae741eb146845d6889a66205a8a
CRs-Fixed: 667453
Signed-off-by: Ravi Aravamudhan <aravamud@codeaurora.org>