omap: nand: remove hardware ECC as default

Commit 2c01946c (omap3 nand: cleanup
virtual address usages) wrongly enabled CONFIG_MTD_NAND_OMAP_HWECC
which breaks boards like beagle and pandora that use software ECC
for write.

Boards like beagle and pandora uses sw ecc for write (e.g. binary flushed
from u-boot) and read from kernel.

Signed-off-by: Sukumar Ghorai <s-ghorai@ti.com>
Acked-by: David Woodhouse <David.Woodhouse@intel.com>
[tony@atomide.com: updated comments]
Signed-off-by: Tony Lindgren <tony@atomide.com>