mtd: nand: omap: ecc.calculate: merge omap3_calculate_ecc_bch4 in omap_calculate_ecc_bch

merges omap3_calculate_ecc_bch4() into omap_calculate_ecc_bch() so that
common callback can be used for both OMAP_ECC_BCH4_CODE_HW and
OMAP_ECC_BCH4_CODE_HW_DETECTION_SW ecc-schemes

+---------------------+-------------------------------------------------------+
|ecc-scheme           | nand_chip->calculate() after this patch               |
+---------------------+-------------------------------------------------------+
|HAM1_ECC             | omap_calculate_ecc()                                  |
+---------------------+-------------------------------------------------------+
|BCH4_HW_DETECTION_SW | omap3_calculate_ecc_bch4() -> omap_calculate_ecc_bch()|
|BCH4_HW              | omap_calculate_ecc_bch()                              |
|BCH8_HW_DETECTION_SW | omap3_calculate_ecc_bch8()                            |
|BCH8_HW              | omap_calculate_ecc_bch()                              |
+---------------------+-------------------------------------------------------+

Tested-by: Stefan Roese <sr@denx.de>
Signed-off-by: Pekon Gupta <pekon@ti.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>