cnss2: NULL terminate fw build timestamp string

If fw build timestamp passed by QMI from firmware is a non-NULL
terminated string, it might result in a out-of-bounds read when
driver parses the QMI message. Hence manually NULL terminate the
string.

Change-Id: I3597df1a8f69824695443880b161d028b63f9af5
Signed-off-by: Yue Ma <yuem@codeaurora.org>