fs: fat: write returns error code instead of -1

It would be good that FAT write function return error code instead of
just returning -1 as fat_read_file() does.
This patch attempts to address this issue although it is 'best effort
(or estimate)' for now.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Signed-off-by: Alexander Graf <agraf@suse.de>