dmaengine: sh: rz-dmac: Save the start LM descriptor

dmaengine: sh: rz-dmac: Save the start LM descriptor

Save the start LM descriptor to avoid starting from the beginning of the
channel's LM descriptor list in rz_dmac_calculate_residue_bytes_in_vd().
This avoids unnecessary iterations.

Tested-by: John Madieu <john.madieu.xa@bp.renesas.com>
Signed-off-by: Claudiu Beznea <claudiu.beznea.uj@bp.renesas.com>
Tested-by: Tommaso Merciai <tommaso.merciai.xr@bp.renesas.com>
Link: https://patch.msgid.link/20260526084710.3491480-7-claudiu.beznea@kernel.org
Signed-off-by: Vinod Koul <vkoul@kernel.org>