diag: Expand msg mask buffer dynamically

Diag driver should support any number of msg mask set by the tools.
It truncates the msg mask to the maximum size of the buffer that is
used to store the msg mask. This patch implements this change.

Change-Id: Ieff2180475c5c1bcf9f8a90dffc78d689b2e5c5e
Signed-off-by: Ravi Aravamudhan <aravamud@codeaurora.org>