drivers/video/ps3fb: fix memset size error

The size passed to memset is wrong.

Signed-off-by Li Zefan <lizf@cn.fujitsu.com>
Acked-by: Geert Uytterhoeven <Geert.Uytterhoeven@sonycom.com>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>