dmaengine: sh: rz-dmac: Add helper to check if the channel is enabled

dmaengine: sh: rz-dmac: Add helper to check if the channel is enabled

Add the rz_dmac_chan_is_enabled() helper to check if a channel is
enabled. This helper will be reused in subsequent patches.

Reviewed-by: Frank Li <Frank.Li@nxp.com>
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/20260526084710.3491480-8-claudiu.beznea@kernel.org
Signed-off-by: Vinod Koul <vkoul@kernel.org>