diag: Set the log mask item number correctly

Diag driver was setting the value of item number for a log equipment
id incorrectly. The item number should be set to the minimum of
the number of supported items and the requested items.

Change-Id: I558d4af14c2eefb0c53e432ed9056ca15406aa7d
Signed-off-by: Ravi Aravamudhan <aravamud@codeaurora.org>