Fix misaligned buffer in env_fat_save

When saving the environment on a platform which has DMA alignment
larger than the natural alignment, env_fat_save triggers a debug
message in file_fat_write:

  Saving Environment to FAT... writing uboot.env
  FAT: Misaligned buffer address (9df1c8e0)
  OK

Signed-off-by: Alex Kiernan <alex.kiernan@gmail.com>