[MTD] [NAND] at91_nand rdy_pin fix

The patch below fixes nand driver for AT91 boards which do not have NAND
R/B signal connected to gpio (rdy_pin is not connected).

Signed-off-by: Ivan Kuten <ivan.kuten@promwad.com>
Acked-by: Andrew Victor <andrew@sanpeople.com>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>