fbdev: uvesafb: Convert snprintf to scnprintf

Coccinelle reports: WARNING: use scnprintf or sprintf

Adding to that, there has also been some slow migration from snprintf to
scnprintf. This article explains the rationale for this change:
https: //lwn.net/Articles/69419/

Signed-off-by: Jules Irenge <jbi.octave@gmail.com>
Signed-off-by: Helge Deller <deller@gmx.de>