mtd: fsmc: Improve the fsmc_correct_data() routine

This patch improves the error correction routine for bch8
- Loop only up to number of errors detected
- Improve the error index calculation procedure

Additionally, it also renames the "correct" routine to indicate that it is bch8
specific

Signed-off-by: Armando Visconti <armando.visconti@st.com>
Signed-off-by: Vipin Kumar <vipin.kumar@st.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>