[MTD] [NAND] at91_nand: control NCE signal

This driver did not control NCE signal during normal operations (only
enable NCE on probing and disable NCE on removing).  This patch make
NCE signal inactive on idle state.

Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>