diag: Fix event mask size value

Diag event mask size should hold one byte more than the maximum
events allowed. This patch increases the default event mask size
by 1.

Change-Id: I4716c86d872e7f802b5bc7c71d76ec43c75b7b40
Signed-off-by: Ravi Aravamudhan <aravamud@codeaurora.org>