dfu: unify mmc/nand read/write ops enum

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 <[email protected]>
Cc: Heiko Schocher <[email protected]>
Cc: Marek Vasut <[email protected]>
Cc: Lukasz Majewski <[email protected]>
Cc: Pantelis Antoniou <[email protected]>
Acked-by: Lukasz Majewski <[email protected]>
Acked-by: Marek Vasut <[email protected]>
Acked-by: Heiko Schocher <[email protected]>