diag: Check mask size properly before copying to update buffer

Currently mask_size is being checked against fixed len macro,
which is leading to not copying mask to update buffer. The patch
removes redundent mask size boundary checks.

Change-Id: I3348b98db3f4f0d9b36cb17d4169d72075cd592b
Signed-off-by: Hardik Arya <harya@codeaurora.org>