m68k: sun3: Replace vsprintf() with bounded vsnprintf()

m68k: sun3: Replace vsprintf() with bounded vsnprintf()

vsprintf() performs no bounds checking and can overflow - replace it
with the safer vsnprintf().

Also remove the useless '+ 1' that is a leftover of commit 66ed28ea096c
("m68k: sun3: Remove unused vsprintf() return value in prom_printf()").

Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
Link: https://patch.msgid.link/20260117202152.1036278-2-thorsten.blum@linux.dev
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>