diag: Sanitize non-hdlc pkt length against buffer capacity

Avoid possible out of bound access by sanitizing the non-hdlc
packet length against the buffer capacity.

Change-Id: Ic54c30fd9c6b2985241387760580613267be2654
Signed-off-by: Manoj Prabhu B <quic_bmanoj@quicinc.com>