mtd: spi-nor: Changed the dummy cycles in spi_nor.c

Use dummy cycles,not bytes,to be in sync with mainline.

Signed-off-by: Anurag Kumar Vulisha <anuragku@xilinx.com>
Reviewed-by: Harini Katakam <harinik@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>