diag: Fix feature mask received check

Currently, there is an array index out of bounds condition when
checking if the feature mask has been received from a peripheral
yet. Fix the check to prevent array index out of bounds.

CRs-Fixed: 564309
Change-Id: I38ea1584cdd3121c0981c247fe9ded37881b7f7a
Signed-off-by: Dixon Peterson <dixonp@codeaurora.org>