dfu: unify mmc/nand read/write ops enum

MMC and NAND independently defines same enumerators for read/write.
Unify them by defining enum in dfu header. RAM support that is being
added newly also can make use of it.

Signed-off-by: Afzal Mohammed <afzal.mohd.ma@gmail.com>
Cc: Heiko Schocher <hs@denx.de>
Cc: Marek Vasut <marex@denx.de>
Cc: Lukasz Majewski <l.majewski@samsung.com>
Cc: Pantelis Antoniou <panto@antoniou-consulting.com>
Acked-by: Lukasz Majewski <l.majewski@samsung.com>
Acked-by: Marek Vasut <marex@denx.de>
Acked-by: Heiko Schocher <hs@denx.de>