diag: Make fixes in memory device mode

When there are multiple clients voting for memory device mode, diag
driver is incorrectly picking the client for draining data. Also,
memory device client that is currently receiving data won't receive
data when a new memory device client comes up. This patch addresses
these issues.

Change-Id: I6c2b3f46a1ade60c58db9c35b551f490268cfbee
Signed-off-by: Ravi Aravamudhan <aravamud@codeaurora.org>