mtd: spi-nor: fix for bank cross over write issue

Changed the spi_nor_read_ext for bank cross over issue while programming
data to the flash

Signed-off-by: Anurag Kumar Vulisha <anuragku@xilinx.com>
Reviewed-by: Punnaiah Choudary Kalluri <punnaia@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>