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>

drivers/mtd/nand/raw/Kconfig

@@ -22,7 +22,7 @@ menuconfig MTD_RAW_NAND
 if MTD_RAW_NAND
 
 config MTD_NAND_ECC_SW_BCH
-	tristate "Support software BCH ECC"
+	bool "Support software BCH ECC"
 	select BCH
 	default n
 	help

drivers/mtd/nand/raw/Makefile

@@ -2,7 +2,7 @@
 
 obj-$(CONFIG_MTD_RAW_NAND)		+= nand.o
 obj-$(CONFIG_MTD_NAND_ECC_SW_HAMMING)	+= nand_ecc.o
-obj-$(CONFIG_MTD_NAND_ECC_SW_BCH)	+= nand_bch.o
+nand-$(CONFIG_MTD_NAND_ECC_SW_BCH)	+= nand_bch.o
 obj-$(CONFIG_MTD_SM_COMMON) 		+= sm_common.o
 
 obj-$(CONFIG_MTD_NAND_CAFE)		+= cafe_nand.o