fs/fat/fat.c: fix warning: 'part_size' defined but not used

Commit c30a15e5 "FAT: Add FAT write feature" introduced a compiler
warning.  Fix this.

Signed-off-by: Wolfgang Denk <wd@denx.de>
Cc: Donggeun Kim <dg77.kim@samsung.com>
Cc: Kyungmin Park <kyungmin.park@samsung.com>