diag: Handle event enable command properly

Currently, all the events remain enabled on
receiving event enable command since tool sends
enable all events command followed by specific
event enable command. Diag is not handling event
enable command properly due to which all the
events are getting enabled. The patch fix the
issue by adding proper checks.

Change-Id: I204779a398af6cfcccc5042077f26d30fc21946d
Signed-off-by: Mohit Aggarwal <maggarwa@codeaurora.org>