FAT write: Fix compile errors

This patch removes compile errors introduced by
commit 9813b750
'fs/fat: Fix FAT detection to support non-DOS partition tables'

fat_write.c: In function 'disk_write':
fat_write.c:54: error: 'part_offset' undeclared (first use in this function)
fat_write.c:54: error: (Each undeclared identifier is reported only once
fat_write.c:54: error: for each function it appears in.)
fat_write.c: In function 'do_fat_write':
fat_write.c:950: error: 'part_size' undeclared (first use in this function)

These errors only appear when this code is enabled by
defining CONFIG_FAT_WRITE option.

This patch was originally part of

http://article.gmane.org/gmane.comp.boot-loaders.u-boot/121847



Signed-off-by: Donggeun Kim <dg77.kim@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Maximilian Schwerin <mvs@tigris.de>

Fixed patch author and added all needed SoB from the original patch
and also submitter's SoB. Extended commit log.
Signed-off-by: Anatolij Gustschin <agust@denx.de>