mtd: m25p80: Revive dual read support

Commit 03e296f6

 ("mtd: m25p80: use the SPI
nor framework") accidentally removed support for Dual SPI read transfers.
Add it back.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Huang Shijie <shijie8@gmail.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>