mtd: seama: fix image data handling

1) Put sanity checks in one place
2) Respect provided offset
3) Read only as much data as needed for MD5 calculation

Thanks to the last change this is a great speedup and memory saver. On
devices with NAND flash we were allocating & reading about 128 MiB while
something about 8 MiB is enough.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>