mtd: rawnand: fix build dependency

mtd: rawnand: fix build dependency

When enabling CONFIG_MTD_NAND_ECC_SW_BCH as a module, the
MTD_NAND_ECC_SW_BCH depends on MTD_NAND, but the module controlled by
MTD_NAND links against the module controlled by MTD_NAND_ECC_SW_BCH.
This leads to the following link failure.

aarch64-linux-gnu-ld: drivers/mtd/nand/raw/nand_base.o: in function `nand_cleanup':
../drivers/mtd/nand/raw/nand_base.c:5886: undefined reference to `nand_bch_free'
aarch64-linux-gnu-ld: ../drivers/mtd/nand/raw/nand_base.c:5886:(.text+0x9928): relocation truncated to fit: R_AARCH64_CALL26 against undefined symbol `nand_bch_free'
aarch64-linux-gnu-ld: drivers/mtd/nand/raw/nand_base.o: in function `nand_set_ecc_soft_ops':
../drivers/mtd/nand/raw/nand_base.c:5093: undefined reference to `nand_bch_calculate_ecc'
aarch64-linux-gnu-ld: ../drivers/mtd/nand/raw/nand_base.c:5093:(.text+0xe914): relocation truncated to fit: R_AARCH64_ADR_PREL_PG_HI21 against undefined symbol `nand_bch_calculate_ecc'
aarch64-linux-gnu-ld: ../drivers/mtd/nand/raw/nand_base.c:5093: undefined reference to `nand_bch_calculate_ecc'
aarch64-linux-gnu-ld: ../drivers/mtd/nand/raw/nand_base.c:5094: undefined reference to `nand_bch_correct_data'
aarch64-linux-gnu-ld: ../drivers/mtd/nand/raw/nand_base.c:5094:(.text+0xe934): relocation truncated to fit: R_AARCH64_ADR_PREL_PG_HI21 against undefined symbol `nand_bch_correct_data'
aarch64-linux-gnu-ld: ../drivers/mtd/nand/raw/nand_base.c:5094: undefined reference to `nand_bch_correct_data'
aarch64-linux-gnu-ld: ../drivers/mtd/nand/raw/nand_base.c:5148: undefined reference to `nand_bch_init'
aarch64-linux-gnu-ld: ../drivers/mtd/nand/raw/nand_base.c:5148:(.text+0xebbc): relocation truncated to fit: R_AARCH64_CALL26 against undefined symbol `nand_bch_init'

Rework CONFIG_MTD_NAND_ECC_SW_BCH from tristate to bool,
and then link the nand_bch.o file into nand.ko if its enabled.

Fixes: 51ef1d0b2095 ("mtd: nand: Clarify Kconfig entry for software BCH ECC algorithm")
Signed-off-by: Anders Roxell <anders.roxell@linaro.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>