Revert "fs: fat: assign rootdir sector when accessing root directory"

This particular commit is causing a regression on stih410-b2260 and
other platforms when reading from FAT16.  Noting that I had rebased the
original fix from Thomas onto then-current master, there is also
question from Akashi-san if the change is still needed after other FAT
fixes that have gone in.

This reverts commit a68b0e11.

Reported-by: Patrice Chotard <patrice.chotard@st.com>
Cc: AKASHI Takahiro <takahiro.akashi@linaro.org>
Cc: Thomas RIENOESSL <thomas.rienoessl@bachmann.info>
Signed-off-by: Tom Rini <trini@konsulko.com>