dmaengine: sh: rz-dmac: Fix incorrect NULL check for list_first_entry()

dmaengine: sh: rz-dmac: Fix incorrect NULL check for list_first_entry()

list_first_entry() does not return NULL when the list is empty,
making the existing NULL check invalid. Use list_first_entry_or_null()
instead.

Fixes: 21323b118c16 ("dmaengine: sh: rz-dmac: Add device_tx_status() callback")
Cc: stable@vger.kernel.org
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/20260526084710.3491480-3-claudiu.beznea@kernel.org
Signed-off-by: Vinod Koul <vkoul@kernel.org>