diag: Fix diag get msg mask for subid invalid case

Currently diag is returning error response to get
msg mask command for invalid subid case. The patch
fixes this issue with using legacy mask for sending
the response to tool.

Change-Id: I85ae460e44f600cc7f7218287a0fc469f7dc2d8f
Signed-off-by: Hardik Arya <harya@codeaurora.org>