mtd: nand: omap: ecc.hwctl: refactor omap_enable_hwecc_bch for ECC related GPMC configs

Lots of if..then..else conditions in omap_enable_hwecc_bch() can be avoided if
code is refactored based on ecc-scheme.

Signed-off-by: Pekon Gupta <pekon@ti.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>