mtd: spi-nor: fix the wrong dummy value

For the DDR Quad read, the dummy cycles maybe 3 or 6 which is less then 8.
The dummy cycles is actually 8 for SPI fast/dual/quad read.

This patch makes preparations for the DDR quad read, it fixes the wrong dummy
value for both the spi-nor.c and m25p80.c.

Signed-off-by: Huang Shijie <b32955@freescale.com>
Acked-by: Marek Vasut <marex@denx.de>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>