[MTD] [NAND] at91_nand: use at91_nand_{en,dis}able consistently.

Use at91_nand_enable(), at91_nand_disable() to manipulate enable_pin.
No functional changes.

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