diag: Send proper response for get mask command

Currently diag is not returning proper response for
get msg build mask command for new multi-sim commands.
The patch updates the response buffer properly before
sending it to tools.

Change-Id: I7507a2094f469b9322d75011e583c9838711f784
Signed-off-by: Hardik Arya <harya@codeaurora.org>