mtd: gpmi: add sanity check for the ECC

We do the check based on the following two facts:

[1] The mx23/mx28 can only support 20-bits ECC, while the mx6
    can supports 40-bits ECC.

[2] The mx23/mx28 can only support the GF13, while the mx6
    can supports GF13 and GF14.

Signed-off-by: Huang Shijie <b32955@freescale.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>