disk: Simplify disk_blk_{write, erase}() using blk_{write, erase}()

disk: Simplify disk_blk_{write, erase}() using blk_{write, erase}()

These two functions are basically identical, just call the blk_*()
functions from disk_blk_*() functions. The only difference is that
the disk_blk_*() functions have to use parent block device as the
udevice implementing block device operations.

Add documentation on what those functions really do. The documentation
is not wrong even though it likely does look that way. The write/erase
functions really do not take into account the partition offset. This
will be fixed in the next patch.

Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>