fbdev: correct buffer size limit in fbmem_read_proc()

Address http://bugzilla.kernel.org/show_bug.cgi?id=7189



It should check `clen', not `len'.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Adrian Bunk <bunk@stusta.de>