mtd: fsmc: Newly erased page read algorithm implemented

A newly erased page contains ff in data as well as spare area. While reading an
erased page, the read out ecc from spare area does not match the ecc generated
by fsmc ecc hardware accelerator. This is because ecc of data ff ff is not ff
ff. This leads to errors when file system erases and reads back the pages to
ensure consistency.

This patch adds a software workaround to ensure that the ecc check is not
performed for erased pages. This problem is solved by checking the number of
bits (in 512 byte data + 13 byte ecc) which are 0. If these number of bits are
less than 8, the page is considered erased and correction algorithm is not tried
on that page

Signed-off-by: Vipin Kumar <vipin.kumar@st.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>