diag: Avoid possible out of bound access while sending masks

Due to mask size wrongly calculated with incorrect range possible
out of bound access is possible. Prevent the possibility by
correcting the range.

Change-Id: I1b8c50411a6fb17f9ca6e90c0646690953f3db0e
Signed-off-by: Manoj Prabhu B <quic_bmanoj@quicinc.com>