mtd: nand: omap: fix compile warning on ‘erased_sector_bitflips’

Commit 2c9f2365 (mtd: nand: omap: ecc.calculate: merge omap3_calculate_ecc_bch4
in omap_calculate_ecc_bch) introduced minor compile warning
"‘erased_sector_bitflips’ defined but not used [-Wunused-function]" when
compiling without CONFIG_MTD_NAND_OMAP_BCH. Move function
erased_sector_bitflips() into the same ifdef section as the only caller.

Signed-off-by: Christian Engelmayer <cengelma@gmx.at>
Reviewed-by: Jingoo Han <jg1.han@samsung.com>
Acked-by: Pekon Gupta <pekon@ti.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>