lib/bch: Rework a little bit the exported function names

There are four exported functions, all suffixed by _bch, which is
clearly not the norm. Let's rename them by prefixing them with bch_
instead.

This is a mechanical change:
    init_bch -> bch_init
    free_bch -> bch_free
    encode_bch -> bch_encode
    decode_bch -> bch_decode

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Link: https://lore.kernel.org/linux-mtd/20200519074549.23673-2-miquel.raynal@bootlin.com