spl: Set FAT bl_len to ARCH_DMA_MINALIGN

spl: Set FAT bl_len to ARCH_DMA_MINALIGN

Instead of relying on the presence of filename to determine whether we are
dealing with a FAT filesystem (and should DMA-align the buffer), have FAT set
bl_len to ARCH_DMA_MINALIGN instead. With this done, we can remove the
special-case logic checking for the presence of filename.

Because filesystems are not block-based, we may read less than the size passed
to spl_load_info.read. This can happen if the file size is not DMA-aligned. This
is fine as long as we read the amount we originally wanted to. Modify the
conditions for callers of spl_load_info.read to check against the original,
unaligned size to avoid failing spuriously.

Signed-off-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>