fs: fat: don't call disk_write with zero sector num

In the set_cluster() function, it will convert the buffer size to sector
numbers. Then call disk_write() to write by sector.
For remaining buffer, the size is less than a sector, call disk_write()
again to write them in one sector.

But if the total buffer size is less then one sector, the original code
will call disk_write() with zero sector number. It is unnecessary.
So this patch fix this. Now it will not call disk_write() if total buffer size
is less than one sector.

Signed-off-by: Josh Wu <josh.wu@atmel.com>