mtd: nand: davinci: Reinitialize the HW ECC engine in 4bit hwctl

This fixes subpage writes when using 4-bit HW ECC.

There has been numerous reports about ECC errors with devices using this
driver for a while.  Also the 4-bit ECC has been reported as broken with
subpages in [1] and with 16 bits NANDs in the driver and in mach* board
files both in mainline and in the vendor BSPs.

What I saw with 4-bit ECC on a 16bits NAND (on an LCDK) which got me to
try reinitializing the ECC engine:
- R/W on whole pages properly generates/checks RS code
- try writing the 1st subpage only of a blank page, the subpage is well
  written and the RS code properly generated, re-reading the same page
  the HW detects some ECC error, reading the same page again no ECC
  error is detected

Note that the ECC engine is already reinitialized in the 1-bit case.

Tested on my LCDK with UBI+UBIFS using subpages.
This could potentially get rid of the issue workarounded in [1].

[1] 28c015a9da ("mtd: davinci-nand: disable subpage write for keystone-nand")

Fixes: 6a4123e581 ("mtd: nand: davinci_nand, 4-bit ECC for smallpage")
Cc: <stable@vger.kernel.org>
Signed-off-by: Karl Beldan <kbeldan@baylibre.com>
Acked-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>

drivers/mtd/nand/davinci_nand.c

@@ -240,6 +240,9 @@ static void nand_davinci_hwctl_4bit(struct mtd_info *mtd, int mode)
 	unsigned long flags;
 	u32 val;
 
+	/* Reset ECC hardware */
+	davinci_nand_readl(info, NAND_4BIT_ECC1_OFFSET);
+
 	spin_lock_irqsave(&davinci_nand_lock, flags);
 
 	/* Start 4-bit ECC calculation for read/write */