raw read/write was not possible because we enabled the ECC engine
during driver initialization. To support raw mode we have to disable
the ECC engine dynamically when needed. This has to be done before
the send_page function is called. The places where we have to disable
the ECC engine are not available in the driver, but are buried in the
Nand layer. To make them available we have to implement driver specific
write/read_page functions in which we configure the ECC mode.
This also makes the driver better readable as it makes the detour around
the driver internal data buffer when reading/writing pages unnecessary.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>