Xilinx: ARM: MTD: m25p80: fix dual qspi bug in flash

The last change that was done to make the dual QSPI
work from device tree only without a kernel config
change had a bug which was preventing the 2nd 16 MB
of flash from working on the ZC706.

Signed-off-by: John Linn <john.linn@xilinx.com>