mtd: atmel_nand: implement the nfc_device_ready() by checking the R/B bit

In nfc_device_ready(), it's more reasonable to check R/B bit in NFC_SR
than waiting for the R/B interrupt. It cost less time.

Signed-off-by: Josh Wu <josh.wu@atmel.com>
Tested-by: Matthieu Crapet <Matthieu.Crapet@ingenico.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>