fs/fat/fat_write.c: Fix GCC 4.6 warnings

Fix:
fat_write.c: In function 'find_directory_entry':
fat_write.c:826:8: warning: variable 'prevcksum' set but not used
[-Wunused-but-set-variable]
fat_write.c: In function 'do_fat_write':
fat_write.c:933:6: warning: variable 'root_cluster' set but not used
[-Wunused-but-set-variable]
fat_write.c:925:12: warning: variable 'slotptr' set but not used
[-Wunused-but-set-variable]

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