fat: fix build warning

This fixes a compile warning (unititialized variable) in
the fat filesystem code.

Signed-off-by: Jonas Aberg <jonas.aberg@stericsson.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>