mtd: rawnand: gpmi: Rename the variable ecc_chunk_size

There is only one variable ecc_chunk_size in bch_geometry data
structure but two different field in BCH registers. The data0_size in
BCH_FLASH0LAYOUT0 and datan_size in BCH_FLASH0LAYOUT1 should have
dedicate variable since they might set to different values in some
cases. For instance, if need dedicate ecc for meta area, the data0_size
should be 0 rather than datan_size, but for all other cases, data0_size
still equals to datan_size and it won't bring any function change.

Signed-off-by: Han Xu <han.xu@nxp.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20220412025246.24269-5-han.xu@nxp.com