mxc_nand: rework get_dev_status

We save/restore the value in the buffer anyway, so it makes
no difference whether we use main_area0 or main_area1. So,
we can use main_area0 and remove main_area1 from the driver
which is otherwise unused. Also, clean up the comments in
get_dev_status.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>