caif: handle snprintf() return

snprintf() returns the number of bytes that would have been written.  It
can be larger than the size of the buffer.  The current code won't
overflow, but people cut and paste this stuff so lets do it right and
also make the static checkers happy.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>