cmd: blk_common: Stop using hard-coded block size for Sandbox operations

commit 3d2fc797 ("cmd: blk: Allow generic read/write operations to work in sandbox")
used the hard-coded block size (512) for accessing the sandbox host
device. Now that we have added support for non-512 block size for both
Sandbox host device and blkmap driver, let's stop using the hard-coded
block size.

Signed-off-by: Bin Meng <bmeng@tinylab.org>
Reviewed-by: Simon Glass <sjg@chromium.org>