mtd: nand: omap_gpmc: remove unused members of 'struct nand_bch_priv'

This patch prepares to refactor 'struct nand_bch_priv' -> 'struct omap_nand_info'
And thus performs following clean-ups:
 - remove nand_bch_priv.type: use nand_bch_priv.ecc_scheme instead
 - remove nand_bch_priv.mode: <unused>

Signed-off-by: Pekon Gupta <pekon@ti.com>
Reviewed-by: Stefan Roese <sr@denx.de>