fbdev: fix snprintf() limit in show_bl_curve()

The limit should be "PAGE_SIZE - len" instead of PAGE_SIZE.  Also let's
use scnprintf() because snprintf() returns the number of bytes which
would have been printed if there were space and scnprintf() returns the
number of bytes actually printed.

I don't think we are ever going to actually hit this limit in real life.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>

diff --cc drivers/video/fbdev/core/fbsysfs.c
Simple merge