sunxi: mksunxiboot: Fix loading of files with a size which is not a multiple of 4

We should not be aligning the amount of bytes which we try to read from the
disk, this leads to trying to read more bytes then there are which fails.

file_size is already aligned to BLOCK_SIZE before being stored in
img.header.length, so there is no need for load_size at all.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Ian Campbell <ijc@hellion.org.uk>