mtd: gpmi: imx6: fix the wrong method for checking ready/busy

In the imx6, all the ready/busy pins are binding togeter.
So we should always check the ready/busy pin of the chip 0.

In the other word, when the CS1 is enabled, we should also check the
ready/busy of chip 0; if we check the ready/busy of chip 1,
we will get the wrong result.

Signed-off-by: Huang Shijie <b32955@freescale.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>