mtd: gpmi: update the bitflip_threshold

The origin code misses to update the bitflip_threshold when
we have already get the right ecc_strength.

The patch fixes it.

Signed-off-by: Huang Shijie <b32955@freescale.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>