mtd: atmel_nand: check NFC busy flag by HSMC_SR instead of NFC cmd regs

Currently the driver read NFC command registers to get NFC busy flag.
Actually this flag also can be get by reading HSMC_SR register.

Use the read NFC command registers need mapping a huge memory region.
To save the mapped memory region, we change to check NFC busy flag by
reading HSMC_SR register.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Josh Wu <josh.wu@atmel.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>