dm vdo: slight cleanup of UDS error codes

No need to increment each UDS_ error code manually (relative to
UDS_ERROR_CODE_BASE).

Also, remove unused PRP_BLOCK_START and PRP_BLOCK_END.

Lastly, UDS_SUCCESS and VDO_SUCCESS are used interchangeably; so best
to explicitly set VDO_SUCCESS equal to UDS_SUCCESS.

Signed-off-by: Mike Snitzer <snitzer@kernel.org>
Signed-off-by: Susan LeGendre-McGhee <slegendr@redhat.com>
Signed-off-by: Matthew Sakai <msakai@redhat.com>